WD40 DESTROYS O RINGS ! Check this by putting WD40 on an old O ring, it will swell and degrade.
To loosen a stuck port, I would try the vinegar as suggested. Another approach would be to put a solution of vinegar and liquid detergent on the crack between the port and housing, wait some hours, then use the strap wrench. I carry a strap wrench during trips as it is often necessary to remove ports.
After a dive trip, I seal my housing and immerse it in a tub of water with vinegar, leave it overnight, then rinse it with water, dry and take it apart. I store it disassembled, with the housing back and front separated, the ports off, so as not to compress the O rings..
When installing, first I clean the O ring grooves, threads and bayonet mounts, the I apply a very light coating of silicon on the O rings.