Seems pretty clear that they need to fix the connection. Whether the problem is at their end of the fiber connection or your end at the ONT, there’s no way of knowing. You need to keep calling them. Insist that they send out another tech.

Most consumer modem-routers speak DOCSIS, not MoCA, so you don’t want to get one. While you could probably find a ISP modem-router on eBay with MoCA, it would be more straightforward to buy standalone MoCA adapters and a WiFi Access Point.

You would connect one MoCA adapter between a LAN port on the Xfinity modem-router and the coax splitter. Then the other MoCA adapter between the coax splitter and WiFi Access Point.

You may want to replace the splitter with one that can pass frequencies up to 1675 MHz. A standard 1000 MHz splitter will work though speeds may be reduced.

Finally, it’s recommended to put a MoCA filter onto the input of the splitter to keep the MoCA signal confined to your house. If your Xfinity modem-router uses DOCSIS 3.1, a 2nd MoCA filter on the modem can ensure that the modem avoids using MoCA frequencies. Not all ISPs provision DOCSIS 3.1 to use MoCA frequencies, so the 2nd filter may not be needed.

I read that devices often have a minimum signal strength (~70db) before they decide to switch

Yes, that’s correct. So long as the AP is using the same SSID and password as the router, devices should roam on their own. It’s odd that this is not happening. It shouldn’t matter, but put the router and AP on different Wi-Fi channels.

Maybe you didn’t change anything on the network but something may have changed on its own. A firmware update could have altered the network settings on a device, such as the MyQ Garage Hub. Maybe it changed its IP address and caused a conflict with the router’s IP address. You could check the IP address assignments in the router or the logs.

Assigning a subnet is something you do on a router or a Layer 3 switch. A Layer 2 switch doesn’t care about subnets as far as switching is concerned.