I had this same problem once when my online pass for this game got revoked after my EA subscription ran out.. even tho I owned this game fully and played mp for years before that. Regular tech support agents were absolutely clueless and useless. I don't even understand what is their purpose.
it was EA_Shepard that helped me and sorted it out.
EA_Shepard can you look into this for iamnuff
this is my old thread where you helped me with same situation 2yrs agoo
https://forums.ea.com/discussions/mass-effect-franchise-discussion-en/old-report-mass-effect-3-multiplayer-not-accessible-redeem-online-pass/9240776