htlc.me is not working for a time now and I needed to test lightning on testnet and signet and I build my own, you can try here:

ln.testnet.coinbin.org

ln.signet.coinbin.org

I will also add testnet4 version but there are not many nodes

source code is not ready for release yet but hopefully in near future.

Let me know if there are features you want to see and I can try add them in the future.