Yes CDKeys.com is a trusted site which offers cheap CD Keys on a lot of games. I have bought plenty. If any key doesn't work, they will not only change the key but will refund for you.
HOWEVER, make sure the key you're buying is NOT region restricted, and works on your region. For example. COD Black Ops 3 cannot be activated in Middle East. There may be some games which won't also activate. (I bought the BO3 key seeing it as Global Key, and didn't read the description. It didn't work, for the reason mentioned above. CDKeys staff were more than helpful in refunding my money - took some sweet time, but refunded nonetheless.)
Hope it helps.
Lahore PG Meetup 2016
Venue: ECLECTIC VIBRATIONS. Address: 91-T,1st Floor Phase 2 DHA (Direction: https://goo.gl/maps/ZQr8g6xixzS2)
Date: 13th of November (Sunday)
Time: 03:00 PM (Try to be there on time to avoid stay behind schedule)
I don't know about this strange error. Game disconnected with no specific reason. I only faced this error with one server and then joined another server and it worked. Like what others are suggesting you could try port forwarding though I don't think it will help much. Enable UPnP in modem settings if it hasn't been done already. Also disable firewall/antivirus if any to check. How long does it to load the map ? Have you tried joining unranked with no pb servers ? Is origin working fine ? Try repairing the installation of the game.