Some people use a portable cellular network router which they can turn on-off when they need it. Some router firmware have also been hacked to change Cellular network identifiers like IMEI, but they would involve you buying multiple data SIMs and switching them every time to switch the other identifiers like IMSI: https://github.com/srlabs/blue-merle

Public WiFi does exist, you could in practice use Public WiFi with Per-connection randomised MAC address and an OS-wide VPN so the Public WiFi network wouldn't see what you're doing. If for some reason you need to hide you are using GrapheneOS then select the captive portal to use Google's server or no server (you would need to find an HTTP-only website of choice to captive you).