First time I hear of someone using OKX as a fiat exchange to be honest, but if you're already using Coinbase, then I would imagine you're from UK, EU or the U.S? If so, then maybe you should look into Kraken.
I guess what he means is he exchange coins into USDT and then use the P2P on OKX to exchange it into local currency.
@OP, I haven't experienced that thing. I am trading on OKX and also have an old account from them since 2015 that I recently migrated to a new account because there are several hacking attempts on my old account that's why I decided to contact them via email and ask if I can make a new account and migrate my old account to the new one. I just don't want to waste my history and activity on my old account; I might need those data in the future.
If you want to talk to their real support, they also reply on Twitter/X:
https://x.com/OKXHelpDeskAre you living in a much more restricted country?
They might detect something suspicious to you that triggers additional verification, like maybe using VPN or receive mixed/unclean coins.