If you have your own business then choose the workcation category. Draw up a contract, and employment certificate hiring yourself and authorizing yourself to work remotely in Thailand. Likely need to submit at least 3 months of personal bank statements where you can highlight your income maintaining balance of the 500K funding requirement.
Once issued the visa has a use before/validity date. You have until that date to enter the country, and then you will be stamped in for however long the visa allows for. For most visas you have 90 days from date of issue to use/enter the country.
The Visa itself has a validity of 90 days from the date of issue, this means you have 90 days to enter from the date of issue before the visa expires, upon entry you were stamped in for 90 days.
It's multiple entry Visa where you're stamped in for 6 months at a time. You can apply for an in-country extension at 180 days, but you would have to bounce out and back after that.
Keyword cooking using the search tool brings up a few schools in the Bangkok area. You need to drill down the threads original post to get school names.
Some African continent nationals face extra scrutiny and processing times at many embassies. You'll need to check thoroughly through the embassy website and see if that's true for Nigerians at the Thai Malaysian embassies. Do you know why they were rejected, and if it was 2 different embassies?