Yes. Coax is very fickle. It’s really easy to cause interference or mess up the signal one way or another. Even the change in temperature over the seasons has a strong effect on signal strength.

Call the ISP to get it fixed. Or if you’re up for the challenge, buy a coax stripper, a coax crimper, and some outdoor rated type-F connectors and fix it yourself.

This is called WAN failover (or policy based routing depending on your needs) and that’s of course the job of a router.

ISP’s gateways plugged into the same unmanaged network switch.

This will not work and will cause issues with your networks performance

You’ll have a rock solid connection to the first AP and then a shitty wireless connection to the router and then a rock solid connection to your ISP

Will it work? Yes. But you’re not gonna get the latency nor speed you think you’ll get on your PC.

They also left you like 6 miles worth of slack for no reason.

Welcome to the world of Coax and why I’m so damn glad I got fiber now.

VLANs work on layer-2. A switch can isolate frames between VLANs and that’s it.

You need subnets and a firewall if you want them to communicate with each other (layer-3).