Pho
Unfortunately for you there is not much to complain about, you just seem to not have read or understood how the bunq subscription and the free trial works.
You can find much, much information here just by searching and reading - also about your current issue:
https://together.bunq.com/d/38725-why-was-i-charged-1799
Of course it's possible that bunq does indeed refund you in certain cases, but it's what you signed up for in the end.
A complaint will not bring you anywhere, just ask nicely would be my suggestion!