I would delete the whole /etc/config/wireless and would let it be automatically re-created in the next reboot. Then you would have the default wifi config and could see if you can get the radios to work with manually configured vanilla config. As simple config as possible.
Possibly some hardware references have changed (PCI path, was it after 17.01 or after 18.06?), or something similar.
EDIT:
there can also be trouble with management frames (802.11w) etc. similar advanced config things. The mwlwifi wifi driver in WRTxx00ACx series is not that great. 