Orbi AC3000 RBK50 has stopped working with Uverse 5268AC router

I got the Orbi 8 months ago. Initially it was fantastic. The set-up wizard recognized the Uverse router and put the Orbi into AP mode. Performance was great. I turned off the wifi on the Uverse router and all was well. 

A couple of months ago, we started to lose connection with some frequency. I was constantly having to cycle the power on all devices. Then service would come back and I'd be good for a few hours or days. 

Recently, it's just ground to a halt. I have tried everything I can think of:

1) Factory reset of the Orbi

2) Put Orbi into router mode and set the Uverse router to DMZ+ mode

3) Put the Orbi back into AP mode and turned off DMZ+ mode on the Uverse router

4) Turned the Wifi back on on the Uverse router. I verified that the Uverse router has internet access. 

When I'm running in AP mode right now:

- orbilogin.net says that the Orbi has good internet access

- Wifi devices connect but have no internet access

- Devices hardwired to the Orbi have no internet access

- Uverse router has good internet access when connected to directly

I'm stumped. What should I do? 

I wondered if I could try designating the Orbi as a cascading router on the Uverse settings. Would that work? How do I do that?