
eero
Pro 6 Series
Easy, reliable, smart home ready; but paid features.

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The things that stink are consumer level hardware and using wifi as a network infrastructure. Most of what you describe is due to those two things - cheap design and construction and iffy firmware for the hardware, and the general inadequacy of wifi to provice a reliable pathway, since it's subject to distance, going through something other than air, interference of several kinds. You don't care about speed - fine - but speed (on a node) is a consequence of weak signal, or interference. So, those two things - speed and reliability of connection are tied together. There are other solutions aside from mesh to reach an outbuilding. Ethernet cable, fiber optic cable, point-to-point radio - all are better than trying to stretch a mesh somewhere far away. In general, when placing mesh points you do not place the mesh where you need the wifi, you place it in a spot that gets a *great* signal from the base router and the clients get a *good* signal. The link is more important than the client. That makes it impractical to use for jumping a bunch of space between two buildings. Eero seems to get the most positivity for pure mesh systems. The plethora of offerings by the major consumer mesh manufacturers (Asus, TP-Link, Netgear, D-Link) shows how they would rather keep pumping out new devices than actually make something you would love to own. In fairness to them, wifi is no way to run a network infrastructure but consumers demand it. Wifi itself was designed as a convenience for portable devices, and convoluting it into network infrastructure was not a good idea, but one driven by the need for sales and consumer demand (plus consumer belief that wifi is somehow better.) u/Scotty1928 makes a good point - I also run UniFi. If I was confronted with your issue (of the outbuilding), I would probably first try an outside AP on the house (outside - and you'd need to wire it to your network.) The AC-M is a particularly good UniFi model for this) and see if you get an acceptable signal at your outbuilding.
It's really not a lot worse that something like Asus, who has a decently robust interface (lots of consumer stuff hides everything behind "simplicity"). The payoff is reliability and a long service life (my oldest UniFi AP is 7 years old, still supported, and still working great.)
Just so you know, mesh doesn't bring roaming to wifi - any APs set up with the same authetication configuration (SSID, passphrase, security method) will allow wifi clients to roam amongst them as needed. Mesh uses what setups like Ubiquiti UniFi and commercial networking hardware use to allow *faster* roaming. UniFi would be my recommendation. It doesn't matter what your brother in law thinks.
UniFi - reliability, self-hosted, no cloud, no subscriptions etc.
UniFi has all the blocking and other features that you'd want. I have not tried it but it now also has ad blocking. At this point, I would never change. It's easy to maintain and upgrade etc. If something does fail, it's pretty simple to replace the component and keep moving. It's got a lot of enterprise type features that I like.
In consumer world - Asus and TP-Link are the better choices. Eero is great hardware has a subscription model for some needed (IMO) features. I would avoid Netgear, D-Link and Linksys - they are not what they once were and have subscription models, sometimes poor support, and varying reliability and quality. You could also consider gl.Inet Flint devices if you're looking for an all-in-one router, they have gained a very good reputation. I agree that a better choice than any of the above would be Ubiquiti UniFi and TP-Link Omada is also decent - it's different than the consumer gear. Reliability is one of the major points of these prosumer setups. I've been running UniFi for 7 years, it's great.
Problem 1 you are using the ISP wifi. Those are usually crap and don't allow you to update settings much. Problem 2 the extenders are like mesh in that they are using your own wifi to resend signals. Info to consider: Are you using 5ghz or 2.4ghz signals? Everyone wants the faster 5 ghz but don't consider that these are more affected by walls. If you can use the 2.4ghz your signal will be better. Second you don't want a bunch of your bandwidth eaten up with "back haul" so you need your remote AP(access point) hardwired back to the router. I am a believer in putting the ISP system in passthrough and using your own router/wifi. Option 1 consumer grade. Asus. Get 2 put one at each end of the house. Run a wire between them, one will be your router, the other will be converted to just be an access point. You can set them up in mesh so it's one wifi network and devices can hop as needed. You cannot buy just an AP from Asus, but can get a better and lower cost device. TP link is also good. I avoid everything else. Option 2 Prosumer Get a Ubiquity unifi system. There can be set up simple or go full on and power a football stadium. For home you can get a single unit as the router or one that is router and wifi AP built in, then get a separate dedicated AP for other locations. You can update the AP s as new tech comes out without replacing the working router. GL
It's the way to go. I have 4 unifi aps in my house, a couple in the garages, a couple outside. All hardwired though, no lossy meshing. No kids and wife complaining about wifi.
Ubiquiti unify is your best bet for a great mesh system. I have it on my small holding and have good coverage over 2.5 acres
Ubiquiti UniFi APs are rock solid for me.
Ubiquiti is great but not the same (very) basic setup that Eero offers. For non technical folks, Eero still a good bet. Otherwise Unifi all the way.

eero
Pro 6 Series
Easy, reliable, smart home ready; but paid features.

TP-Link
Deco XE75 Pro
Great coverage, easy; but unreliable Ethernet, poor app.

eero
eero Max 7
Incredibly fast, reliable; but very expensive, limited control.

eero
eero Pro 7
Fast, reliable; but paid features, needs internet to function.

eero
eero 7
Easy, reliable coverage; but no 6GHz, paid features.