No wi-fi adapter found in Ubuntu 18.04, it was working properly before but stopped working yesterday, Intel Wireless 8260 adapter
Solution 1:
In the end, it was a hardware issue.
I sent the laptop to the shop for a checkup and internal cleanup and they found that the wifi adapter connector was slightly off. Windows was still able to establish the connection, but Ubuntu wasn't. This also explains why sometimes the adapter was detected, but unclaimed, and why sometimes it wasn't detected at all.
So, for everyone experiencing similar problems, a wifi adapter working in Windows but not in Ubuntu may still be a hardware issue.