I'll get the experience points and even rank up. But after a little while I'm back to RANK 5 (Pre-order) again. I lost all the dog tags, even though I'm still using one of the dog tags that I got before...

This has happened to me 6 times already. First I thought it was because my PC kept crashing. Now my ranking is always going from 5 to 6, back to 5, up to 6. Same goes for my experience point.

I can play on unranked server and still gain XP toward my rank right? Or is it only on the ranked servers.

you gain xp in unranked servers but they are lost when you leave the server in order to keep the xp you must play on ranked servers