dxld I tried with a different sim from the same operator to no avail. I don't have another one (from a different operator) that has 4G/5G service unfortunately.
It sounds like you're in a troublesome situation. Unfortunately, getting out may require some annoying experimentation.
The least disruptive test would be going somewhere else to see if you have a tower problem. Next least disruptive might be another carrier's SIM card. After that, maybe factory reset of GrapheneOS, then maybe flashing back to the stock OS. If none of those work, you may have a device hardware problem.