Anyone with Xfinity / Arris S33 / Orbi rbr750 compatibility or issues?

In November, I switched to Xfinity from my local cable provider.  I previously used my Arris S33 modem and my Orbi RBR750 Router with my previous ISP with no problems.  

New Coax Cables (Comcast Technician) was run from the street to my house and from the wall to the Cable box, as well as wall to the Modem.  Only one coax cable coming into the house and a new (comcast tech installed) splitter connected to the two cable lines run within the wall.   New Cat6 cabling use from modem to router, and to any wired PCs.  

Since switching to Comcast.  I have found that my Orbi RBR750 router loses its internet connection.  If I switch the cable modem connection from the 1Gbit to 2.5Gbit Ethernet port on my modem, the modem finds an IP, and works again for a while.  Then at some point, I have to switch from the 2.5Gbit, back to the 1.0Gbit to reconnect and get an IP.  (I know I can only use one of these ports at a time).  Unplugging and replugging into the same port does not resolve the issue. 

The second strange thing that I have noticed (independently of the first issue), is I will occasionally drop to around 100Mbps, rather than closer to the 800Mbps that I am paying for.  If I look at Network Stats on my Orbi Setup page, it sometime shows the WAN Port status as 100M/Full, rather than 1000M/Full.  This usually fixes itself after I reboot the modem.

The above things are things I noticed in the past month, and may help to provide context to my latest issue.

Five days ago, my internet was working correctly  (around 800Mbps), but my TV started showing a RDK-03004 error - "Sorry there seems to be some trouble".  Since the first available service call is this upcoming Saturday (a full 8 days after my TV stopped working), I have spent hours chatting every day with different tech support personnel at Comcast, they haven't figured out what the problem should is.   Yesterday, I went to the Xfinity store, and swapped out my Cable TV box for a new one, but still to no avail.  However as I was trying to setup my cable box - my internet went down (yet it was up prior to this).  I tried to a System Restart thru the Xfinity app, and Xfinity Tech also sent commands when chatting on the phone with him.  Nothing worked.

This morning, the modem was no longer flashing (now solid blue) and the Xfinity app said "let's get started" - so I setup the modem again and the internet worked (wired as well as the Orbi RBR750 wireless connections).  TV still shows the same error after going thru a Restart.   Fast forward 4 hours later, and now my internet connection no longer works.  Chatted with support again, and they were able to get my modem back online with a single computer connected but this time have not been able to get the Orbi RBR750 reconnected (ie, no wifi, or wired connections).

I suspect this combination of components is causing some kind of incompatibility - I'd rather not pay the add'l 1$4/month for Xfinity's modem considering I have a decent network setup already.

Anyone else experience any of these types of issues?  Any help/suggested is much appreciated.

--Tom