Where do I find my iPhone's MAC address for the router?

on iOS 18 you don't have to pick between private and real anymore. tap the (i) on your network, Private Wi-Fi Address, and set it to Fixed. it keeps the same address for that network, so the router filter keeps working and you still get privacy on other networks.

Settings > General > About > Wi-Fi Address is the phone's real one. But careful, by default the iPhone uses a different private address for each wifi network, so if you put the one from About into the router's filter it won't match. Either go to Settings > Wi-Fi, tap the (i) next to your network and use the address shown there, or switch Private Wi-Fi Address off for that network so it uses the real one.