Orbi AX5400 Tri-Band WiFi Mesh System

I just setup a 760 router with 2 satellites.. Every thing is working great. Coming from an Asus router , This has been a huge upgrade for our home . Much faster speeds and great coverage

No upgrading to 2 gig would do absolutely nothing, your problem right now is your Wifi coverage. Orbi's are usually pretty decent but it really depends on the model. If you have the 300 series (Like the Orbi 370) they don't perform that well when they are not hard-wired together (through ethernet) If you have the Orbi 700/900 series try to move them closer together/check connection strength in the Orbi app. If you have ethernet going around the house (homes built from 2006-2019 generally have them, after that they started relying on wifi stupidly) you can make the 300 series work very well. You can buy some cheap ethernet cables and connect them to the Mesh system and you should get substantially better speeds. Keep in mind however you would have to have the infrastructure to get those Ethernet ports to actually be "live" the easiest way to identify if you have this is to see is to figure out where all the Ethernet ports are going (usually in some utility closet) You should see a box that all the ethernet wires connect into, if you just see wires your going to need to be a "Gigabit Network Switch" with the amount of ports needed, If it's not near your ont your going to have some issues, but you might be able to send ethernet back down if you have an ethernet near your main Orbi base. If you don't have Ethernet you should upgrade to a substantially better Mesh system that uses different antennas for the backhaul (connecting the routers to the extenders) these are generally pretty pricey ($500-$700 like for the Orbi ) but they might be the only way. Now if you are willing to give Verizon a shot your plan probably includes "whole home wifi" where Verizon will figure out the extender situation and make sure you have a good connection in most places but you'll have to use their equipment. As for that "white box" I don't really know what that could be unless it's an old power supply for the ONT (does it have a power connector on one end?) Just a quick thing the "black modem" is called an ONT it essentially does the same thing as a "modem" on a cable provider (Take the Fiber coming in and convert it the protocol + connector to ethernet). Also out of curiosity how big is your home, that may be causing some issues

I hated the WiFi 6 Orbi system I bought. It was lacking so many fairly basic features, such as QoS, firmware updates breaking basic functionality, and a very lackluster app-focused experience.  I replaced it with an ASUS WiFi 7 system comprised of a RoG router with two Zen nodes and it’s been a lot more solid with far more configuration options than I’ve ever seen on a residential router.  Maybe the Orbi WiFi 7 line has solved their earlier issues, but I’ll never buy one again based on my prior experience. 

I’ve used before netgear Orbi mesh WiFi 6 router with two satellites. Worked well. Expensive but I guess was worth. Still use it now in the apartment but with one satellite in the bedroom only just in case

You could get a 3 pack mesh network bundle like the Netgear Orbi system with a dedicated wireless back haul. You would plug the main unit in behind your ISP router and then place the satellite units around your living space. Just make sure that you place one of the satellite units mid way between your deadzone and the main unit so it can throw the signal the rest of the way for the last unit.