Correction. If you take your cancellation letter to immigration, you have to leave by midnight that day. You can ask for a further extension that immigration can deny and give you 7 days to keave
you more than likely came in visa exempt. This you can extend one time for 30 days. Transfer your
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0thb to your Thai bank account then apply for your non o based on Marge 2000thb . This gets you 90 day. Before the end of that 90 days apply for your one year extension 1900thb
If you leave Thailand, you will have to apply for a non B visa all over again, you don't just get another 90 days (unless you're on one of those multi entry non b's that haven't been issued for a while). I'd get on to your employer and get them to sort out the paperwork pronto... it doesn't take that long
best of luck. Hopefully you get it,then by end of March Vietnam MAYBE open and you Maybe can get a multiple entry non o in HCMC (they didn't use to bother with financial requirements) a lot of maybies I know