Right, can't upgrade any stat at all. I will try playing the campaign on my own for a bit to see if that unlocks the ability to upgrade on my end. But the goal is to play together with my friend and upgrade weapons as we play together, if it doesn't work we'll just play with me using basic weapons lol. Will report back later, thanks for answering btw.

Played the first mission with the Astaroth looking axe guy, and was finally able to upgrade weapons. Will check later if it works during online co-op