That's a Wasteland problem, not an Arma problem.

Old Arma problem. I can almost guarantee that RMB, / [NUM] or 0 [NUM] will work.

I have a Thomson router and play Arma 3 online with no problems. However, I have had problems with Steam games before. I downloaded some custom files which fixed the problem. I have no idea whether or not this actually works with Arma 3 as well, but it's worth a try. Just google "Client dropped by server" and the top link (Steam Support) should be the one. Just follow the instructions in the green box. If anyone can confirm if this works or not, it would be much appreciated.

I think putting everything on low puts more stress on your CPU. Playing on higher settings might actually give you better framerates.