Unfortunately it won't work as easy as I hoped. Either the hex string is to long or I get a error message that it doesn't belong to the wallet. Guess I'll wait or dig into it in a few days with more time and patience.
Discussion
It worked now, managed to bump fee successfully.🎉
My problem was, that the closing tx didn't show up in the on-chain transactions tab. So I tried to put the closing txid from the channel tab into the speed up tool. Now I know that was wrong.
After I putted the txid from the on-chain transactions into the tool it worked.
Thanks again Evan!
You’re welcome