D2. Cant Join friend in "other multiplayer."

First off, this is my first time using multiplayer in Diablo 2. Also, I do not know if this is put in the right section of the forums. Sorry if I messed that up.

Me and a friend can join each other through battlenet at the main screen, but I want to be able to play with my singleplayer characters. Is that even possible to use my characters I spent a lot of time on from single player, and play with them with my friend?

I known in “other multiplayer” I can host a game through battlenet there and choose those characters, but it fails to let my friend join me. Same if I join him. Fails to join.

Am I not understanding something here? Is the “other multiplayer” option, and go to battlenet there, not used to play with your friends online, in another state? We both play patch 1.14d of LoD. I even have my firewalls off. I honestly don’t want to start over with new characters through the main menus battlenet, because that’s what that does. I am playing new characters from level zero this way. I just want to play with the characters I already created through single player. Can someone let me know if that is even possible, please?