I am actually having the exact same problems. I purchased an XR700 for my Dad for Christmas while I own the XR500 myself. XR500 is back in my condo and works fine (outside of the one weird anomaly saying failed to load back in December, fixed by reboot). But this XR700 has been a nightmare to get running perfectly.
Specific R-App Programs fail to load after the router has been running for a set amount of time after reboot. After resetting the router, I do see the R-App successfully running but it will eventually fail. I've tried downgrading firmware to .10 but no luck. I have not tried factory resetting the router, but I have noticed some weird anomalies. Mainly, I noticed the "Update" button in the top right lights up after the R-App doesn't load, saying there is an update to .20, which I have installed already.
The R-Apps are the same: The Download/Upload Monitor (Network Monitor), A window in Geo Filter, A Window in QOS, and Device Manager. Other APP's, such as CPU usage, work fine.
Mainly, the problems I feel have to deal with the router's ability to track devices internally. After a period of time, it fails but only on the apps that monitor traffic.
The devices I have in the home range from a number of computers, iPhones, Pixel 2, Nintendo Switch, PS4's, etc. We also have two remote Access Points connected via Ethernet in other corners of the home (It is a big home with thick walls), which consist of two routers turned access points (a really old Netgear router and an Almond+).
I do want to mention I also have Fiber (Frontier) for the XR700.