Hmm..
Can you go to https://aternos.org/players and go into 'OPs' and see if you're in that list?
Make sure to do it when the server is online. If you aren't in the list, then add your name.
If it still doesn't work, then it could be a problem with 1.17 (a bug?)
Already did this, and it both shows me online and as an OP. I'll just consider it a bug, then. No creative mode to restore lost gear, then. lol Small price to pay, I suppose.
Thank you for attempting to help.