I had the same problem and tried a lot of different things, then this worked:
Search for the folder where Daedalus Testnet is installed, like: C:\Programs\Daedalus - Rewards V1. Inside this folder, edit the file called jormungandr-config.yaml inserting this: {“address”:"/ip4/185.164.6.221/tcp/54322",“id”:“64e8a1acfd065102fe268fd874f00e910e1794eb49ec71d6”} in the section “trusted peers”.
Then try to sync again. This should work. You can find more details about this solution here: https://forum.cardano.org/t/reward-wallet-stuck-connecting-to-network/