S_Zizzle First of all, make sure the SIM is fully registered and activated with the network. You could call or message their support if you're unsure. They might also be able to suggest other fixes, no need to tell them you're using GrapheneOS. You may also want to check with them if your phones IMEI is blacklisted, it could have been previously stolen.
If that doesn't work then test if the SIM works on stock. If so, maybe make a note of the APN settings, just in case they're different to what you've tried so far. Then try going back to GrapheneOS.
Otherwise try another SIM and network to rule out hardware issues with the modem. You could also request a replacement SIM from ID, in case the one you have is bad.