Daily reboots needed on C7800 when used with EX7700

Been using the C7800 for a little over a year now. Introduced the EX7700 about 11 months ago as our apartment is about 700 sqft but has a long hallway running between the living rm and bedroom. The C7800 sits at the far end of the living rm so when at the far end of the bedroom we need a little bandwidth boost for zoom calls & HD streaming. The house is a 3 family (built about 1907) and is just one of those old weirdly built new england houses where the connection just doesn't reach and the bandwidth drops drastically at extreme opposite ends of the house)

Issue is whenever we're using the EX7700 along with the C7800 for some reason it makes the 5Ghz network drop (either devices just fail to connect or sometimes the 5Ghz network (usually the network not the EXT network) just dissappears from the network list on the device entirely) pretty much daily and requires a power cycle of the C7800 to get connection up & running again.

I have the EX7700 in extender mode (not AP mode) and do have the 2 networks seperate (one wifi mode not enabled) because I prefer the 2.4 and the 5Ghz networks to be seperate on the C7800.

Its only the 5Ghz connection that seems to drop. I thought it was something maybe with the C7800 or the ISP's connection itself so I removed the EX7700 from the equation for troubleshooting purposes, and lo & behold, have not had a drop for almost 2 weeks. So its definitely the extender that's causing the issues. I'm just not quite sure why. 

Could the EX7700 being too far away from the C7800 cause this behavior? Its not halfway between but maybe like 2/3 of the way (because exactly halfway would be in the hallway & the hallway doesn't have a place to put the EX7700 and I was trying to avoid an unsightly cable and mounting it on the wall). If this is the issue, maybe trying to use the EX7700 in AP mode would work better?

I've tried factory resetting both the EX7700 and the C7800, tried setting up the EX7700 with WPS and also with ethernet cable. I've tried all the previous firmwares on the EX7700, including the current one. I've tried not extending the 2.4Ghz at all & just using the EX7700 with the 5Ghz band, that doesn't help either.

I don't really have enough expertise about network topology and/or networking in general to figure it out. I'm kinda ready to ditch the EX7700 and just get an extender that plugs directly into a power outlet, but I know the EX7700 is a good little extender when it works, so I figured I'd post here first. Any help is appreciated and hollar if you need any more details about my configuration.