After cancelation of Work permit, you need to cancel your Visa immediately. Ask HR for the paper about WP cancelation, you will need it for closing visa.
Your agency should have explained you all the process. Now after canceling your NON-B visa you can apply for Covid extension, then your agency will have more time to do new Visa. But usually NON-B visas under BOI only get 14 days after cancelation. Normal NON-B gives you just one free or 7 days for 1900 thb.
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