Same problem here. Not sure if it affects everybody in recent months, or just some users?
I've not played any multiplayer Arma for some months. A friend and I just tried to play a co-op but could never see, nor connect to each other, no matter who hosted or how the connection was initiated. Same as reported above. Public servers were visible, but no amount of waiting for filtering caused ours to show up for the other person. No changes to our PCs recently or ISPs.
So we switched to a completely different game and had no problems at all.
Neither of us have mods installed (well not activated that we are aware of, just the 5 base packages in the Expansions menu).
The only odd thing I noticed was that after a few minutes of trying to host/join a game, my connection became super slow. Publics servers would only appear one at a time and I also couldn't browse the web, but TeamSpeak was unaffected. Maybe my ISP didn't like my PC pinging thousands of servers and blocked/throttled some types of traffic such as port 80? Not sure but it was repeatable and rather weird. Possibly not related but thought I'd mention it.