Once you're a tax resident, foreign-sourced income (like salary from a foreign job, dividends, rental income abroad, etc.) becomes taxable if you bring it into Thailand in the same year you earned it.
For example, you earn £50,000 in the UK in 2025 and transfer it to your Thai bank account in December 2025, it is taxable in Thailand.
However, if you earned that same £50,000 in 2024 and only transferred it to Thailand in 2025, it’s not taxable, even if you’re a tax resident in 2025. Thailand taxes foreign income only if remitted in the same year it's earned.
If the money is legit, take proof with you to the bank and they'll release the funds immediately. If the money isn't legit, then the bank is doing their job properly and you should probably be leaving the country.
"investment in Thailand".. this is an oxymoron. You don't 'invest' in Thailand. You spend money here, with no expectations of ever seeing a real return. As the old saying goes; invest in home.. spend in Thailand.