Our backup/rpc node https://engine.ryamer.com/ can be used on https://nftm.art (though from time to time we switch to the one provided by engine's team when we are doing upgrades/taking backups). The backend for NFTMart is fully powered by our rpc node. Some witness nodes block off their RPC port so you can't get data out of them. Its like hive nodes, where fullnodes can be used for most reasons, witness nodes usually can't.