I desoldered the old, fixed antenna, and replaced it with a removable antenna.

I tested it out, and it works fine, if not better than the old one. Even though “better” is subjective, signal strength to my other router went from -87dBm to -53dBm, both with -99dBm noise.

Note, I had to mount the antenna in a different hole than the previous one, because there were parts in the way of the antenna mount.