CBR750 Wireless devices connect, but no internet

While I somewhat understand the sentiment, I do feel like we're pivoting from the question posed. If the network as connected was altogether non-functional, I'd just pack my things and go home.

However, Orbi does/did function as a router for about 80% of the connected devices without issue, so I'm more focused on what the problem with the other 20% not sending/receiving traffic is, rather than how to replace the whole system. I understand there isn't a manufacturer manual for this, which is why I'm hoping to find some community knowledge here.

I'd suggest a parallel: a manufacturer won't suggest the best way to place a towel under a door to stop the heat from escaping, but if all I have is towels and I put them under 4 doors and it worked perfectly, but the heat is still leaking from the 5th door, I'm not looking to go buy 5 new doors, but to figure out what's keeping the 5th towel from stopping the heat escaping. If it worked 0/5, I wouldn't bother asking for advice.

I get that throwing money at the problem is *a* solution, but if there are a couple of software tweaks (opening ports, checking device permissions, changing DNS server are some I've considered), that would be *the* solution I'm looking for. 

PS: I did swap my R7000 Nighthawk for now, to handle the rest of the devices in the half of the house AT&T router can't reach, but it's clumsy with dead spots and buying a $400+ replacement doesn't excite me. I'd rather try my luck with a different manufacturer if that's the route I'll have to go.

Cheers and Merry Christmas!