Just a note, GTAO uses a peer to peer system so if you say it lags in a full session, it is possible the host you are connecting to that is causing it [their upload is getting saturated?].
To further test this theory, host a lobby by going into a closed friend session and hopefully you have enough friends to join and fill it up. If there is still lag, something is up and it isn't the international host. See below for where to get accounts to test.
I play on PS3 and i get no trouble on smaller lobbies but more often i get kicked and put alone [apparently the system does this if it thinks you will lag the session? dunno read a comment on reddit saying this] but anyway, i mostly host a session with my cousin and it is more or less smooth sailing.
I won't lie, because of exchange congestion so playing at night is out of the question, i have a more stable connection via Telkom Mobile 3G.
Don't beat yourself up about NAT and DMZ. If it works fine in other games, it isn't your config. Lastly, test with the free 1GB accounts from Webafrica,Afrihost,Axxess,CrystalWeb and if possible MWEB.
One last thing, try setting your DNS server on the PS4 to Primary: 8.8.8.8 & Seconday to 8.8.4.4 to see if anything improves and this can be found under your network settings on the PS4. It helped my cousin.
