Replaced U-Verse with ATT TV

We had ATT U-Verse installed in our new house in 2016. It was bundled with Internet and phone service. The installation used Fiber to the Premises. The house was wired with RG-6 coax but no Ethernet cables. The Pace 5268AC Router/GW was installed. The Router/GW was connected to the ONT via an ethernet cable that the installer put in. The Router/GW coax port was connected to the home coax and splitters to each room to use HomePNA. A mix of RG-6 coax connected and wireless STB's were installed. We put up with signal issues on the wireless STB's for a while and I eventually changed them out with wired coax STB's. Fast forward to 2021 and we changed from U-Verse to ATT TV. All of the U-Verse STB's were returned to ATT. The ATT TV STB's are wireless or ethernet only so the coax (HomePNA) was no longer needed. However, if I disconnect the coax port on the Router/GW, all the lights go out and we loose internet. My question is why? I see no RG-6 cable going to the ONT, only the ethernet cable to the Router/GW. I looked in the Router/GW settings and HomePNA is still enabled but not sure if that is the issue. As far as I know, the only RG-6 coax is internal to the home and all of them should not connected to anything. My plan was to reuse the RG-6 coax to distribute Ethernet using MoCA but can't do this if the Router/GW requires it to be connected. Any help is greatly appreciated.