I guess you're aware that to work legally in Thailand you need:
- A Thai Work Permit (WP) and the primary item needed for a WP is a contract of employment from a Thai registered company. This is granted, administered, renewed by the Thai Labour Ministry. (This is permission to work in Thailand only. It's not permission to physically be in Thailand, that's a different subject.)
- A visa granted, renewed by a different / separate government agency, the Thai Immigration Bureau, gained through having a WP. (This is permission to physically be in Thailand with dates.)