Low signals on WIFI / Bluetooth / GPS / Cellular

I see it's been a couple of months with no answer for you, so I'll see if I can at least offer some insights for you, @xpozza.

Your situation isn't straight-forward, as the antennas you mention are all pretty much separate so it's hard to figure out how they could all go bad at once.

This part is referred to on some sites as the main GSM antenna, located at the bottom of the phone.

And on that same site, this is called the secondary GSM antenna.

The WiFi antenna should be this part here at the top of the phone.

This part is called the bluetooth antenna.

I checked the video you referred to, and the main GSM antenna is the part they're showing you how to replace. Outside of getting the screen off, it shouldn't be a particularly difficult repair, so it would be worth a try. Although there isn't a guide specifically for that part here on iFixit, if you follow the instructions on the iPhone X Lower Speaker Replacement guide and stop after step 23, you'll have everything you need.

Unfortunately, I really have no explanation for why all your antennas should fail at the same time. It kind of sounds like something is shorting things together that shouldn't be connected; i.e., the various antennas. It really sounds like you need to have someone take a look at the repair that was done previously and verify that it was done correctly, because it sounds to me like it wasn't.