I know that I can set up paytxfee, but I want that bitcoind server set it up automatically - how can I do that ?
You have always to choose your transaction fee by yourself. There is no "bitcoin server" (neither a "bitcoind server").
The fee of 20 Sat/B appears to be the standard value in your client.
Next time, check
https://bitcoinfees.21.co to see how big the fee has to be to get your TX confirmed within x blocks.
Ant the next time you forgot to change your fee you can use a free TX accelerator to get your transaction faster confirmed.
You can find those in the service section on this forum.