Bought a TP-link AC1200 wifi adapter long before and not used it so far. Yesterday, I find it again when I collected my old stuff. Since my old Dell laptop does not support 5G channel, and its built-in wifi adapter sucks, why not have a try!!!
Searched and tried a lot, and finally, I find the
correct one(which works for me).
The link is: https://ubuntuforums.org/showthread.php?t=2340769
# Uninstall rtl8812au-dkms if you installed it as other solutions told you