Actually it seems you can't play online with a mod.pack installed that is set to status MOD, but you can play with mod.pack, that is set to status PATCH. This is weired, as imo this version check should prevent cheating in online games, but actually it just prevent playing with mods made within the structure supposed by CA, while cheaters can still do what they like as there is no check for moddified or additional patch.packs.
So you might need to search for mods, that are tested online, but take a look, if you have to replace your original Patch.pack or not. Doing this could cause some trouble with updates. There is another opportunity: search for mods, who just add a new pack to the game. This way you can't use mod manager, but your game can update normally, you can activate the mod by adding to or deleting from the data folder and you can play-online. As you would be able to play aslo with unmodded games and this might cause some trouble about cheating complains, you should just use mods with others, who also have them and not in rank battles. If you just use optical mods, you should announce it when joining a battle with others and ask if this is ok.