I’m very pleased you have resolved your problem. I’ve used Bangkok Bank to help with many issues and they’ve always been 100% positive. Maybe worth considering a change of bank?
I used Transferwise 3 months ago to send funds for property purchase, I needed similar bank certificate to prove external source of funds. My Bangkok Bank accepted proof of my Transferwise transaction as I could show identical amounts and they gave me required certificate. May be worth trying to push your Thai bank.