I'll be using mine in an RV, to have one thing for all devices to connect to-- while the router connects via wifi to multiple internet sources, depending on situation (i.e. cellular modem, starlink, phone hotspot, campground wifi, etc).
Will I loose bandwidth/speed when the GL-ATX1800 is in between the source and device?
Is there any way to have the router connect to the best/fastest available wifi source?
Just to add to what was previously said, you can change the priority of the ports in use (ethernet, tethering, repeater) to use the best provider. It just won't do that automatically. You speed may not be much slower at all, even using wifi, if your throughput is low (<100mbps) as the wireless and repeater will certainly be able to keep up with it. It may or may not be noticeable, you will just have to test and see.
Maybe. The trick is, to connect to your source on one band, (probably 2.4GHz) and connect your devices via the router wifi to the other band (5GHz) within your RV. This way, you won't lose half your bandwidth.