Issues with MSFS2020

I've narrowed down some connection problems (failure to get past initial screens) with MSFS2020 to my Orbi RBR40 router.  If I disconnect the router from the Spectrum modem and connect my flight sim PC directly to the modem, all is well.  Going back to the default config: modem->RBR40->all connected devices including the sim PC, the sim PC will fail to execute properly (1 of 5 simulator executions actually works).  If I reboot the router, it will work for a while, then crap out!  I also get a ton of DoS tetxts in the Orbi log...

I've tried numerous settings including setting up a DMZ and disabling DoS protection - nothing made a difference.  Getting ready to junk it and go to another vendor.

Any ideas?  Yes, the firmware is up-to-date.

John