Simple Bitcoin Wallet from https://lightning-wallet.com/ seems the best current Bitcoin+Lightning wallet for Android-only and it may be easy to add Signet support since Mainnet and Testnet are already supported.
For iPhone (it seems, tested just on Android as this is multi-platform smartphone app) BitPay can use Testnet already so again it may be easy to add Signet. But note it is an ugly "crypto" wallet confusing any newbies so it may require a maintained patchset. https://github.com/bitpay/wallet
Blockstream Green wallet has Testnet support on Android, but I heard the recent version does not support Testnet on iPhone. Can anyone verify it please? (no iPhone here)
https://coinos.io is a wonderful web-based wallet, but is there any Testnet/Signet version of it running?
Thanks everyone who up-voted this thread when it was fresh and and noticable just in the "recent" thread list.