Huh. I could have sworn I've seen a video specifically about all those antenna contacts, but cannot find it now
Anyways, the video you posted is also OK, in that it shows how to disassemble the phone. You don't need the full disassembly, you only need to remove the back cover and disconnect the battery (just in case).
You'll find the little springy wire contacts along the perimeter and I think another 2 or 3 in other places, they are hard to mistake for anything else. Clean them gently, maybe also pull upwards a little (*very* gently!). Clean the corresponding contact spots on the inside of the back cover too.
(The hardest part about all this is actually removing the bloody back cover. Videos don't do justice to how much of a bother that part is.)