You could either leave everything where it is, and run another WiFi AP at the other end of the house, or if you have enough data outlets, move the router to a better location, patch the ONT through to the WAN side, and patch the LAN side back to the switch in the garage.
Get a device that's a switch and WAP, plug it into one of the ports in the house, then plug the device that the connection is meant for into the switch. That will give them WiFi in the house. Instead of a combined switch/wap you could get a cheap switch and a decent WAP. I use a cheap DLink router, configured with DHCP off.
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If you add another AP you can set it up easily on the same network and also have wireless devices pick the one with the best signal. Apple airport express is a solid unit and pretty easy for non technical people to set up or Ubiquiti gear for the more technically capable.