Probably the problem is that the DNS seed in your config file are outdated since the wallet is no longer maintained. Meaning, your node has not a single node it connects to to ask for peers in the network.
Not sure if it works, but you could try adding any of the following dns-seed urls as preferred peers in your grin_server.toml.
peers_preferred = [“mainnet.seed.grin.lesceller.com:3414”, “mainnet.seed.grin.prokapi.com:3414”,“grinseed.revcore.net:3414”,"“mainnet-seed.grinnode.live:3414”,“mainnet.grin.punksec.de:3414”, “grinnode.30-r.com:3414”]
Probably that fixes the issue since these seed nodes are nearly always online and can connect you to the rest of the network. If it does not try restoring with another wallet software.
Niffler is just a GUI version of the old grin-wallet (Rust). So alternatively you can always restore the wallet using the rust wallet, IronBelly mobile wallet (under the hood also grin-wallet) or Grin++.