Motorola MB8600 and Orbi RBR20 not getting along

I read a similar post regarding the MBR8611, but I haven't seen anyone post about the 8600.  Long story short, I've had the RBR20 for a number of years and it's worked fine.  In December I upgraded the modem to the 8600 and ever since then my internet connection has dropped once or twice a day.  All of the lights on the modem are fine, but there's just no WIFI internet.  If I reset the modem everything's fine.  

I can't test the LAN connection because if I move the ethernet cable from the Orbi to my computer when the internet isn't working, I need to reset the modem to get it to connect to the desktop, which always solves the problem. 

I've had Xfinity techs come out twice.  Both times they found some interference on the line and did something to correct it, but the second tech told me that the Xfinity logs are showing that the modem has been up for 20ish days, when I know that can't be the case.  

I never thought it could be the Orbi since it worked fine with my old modem.  But today, when the internet dropped, I tried connecting to the modem's IP address to pull the modem event log and it couldn't connect.  So that tells me that there's a communication issue between the Orbi and the 8600 that goes beyond just the internet connection.  

Has anyone figured out which device is to blame here?  I'm outside the return window on the modem, but I did just buy an Arris to give that a go since this entire time I thought the modem was to blame.  And my firmware on the Orbi is updated.  Thanks!