finitefutility Previously the sim card works immediately in another Pixel with stock OS, and another sim did not work in my phone.
finitefutility I tried both a factory reset and have gone back to stock OS, both did not make a difference.
finitefutility I don't think it's carrier related as my sim works in 2 different phones immediately?

If you have tried your SIM card in multiple phones, and it doesn't work in only your 4a5g, and you have tried a different SIM in your 4a5g but that doesn't work, and you have tried two different OSs (GrapheneOS and the stock Pixel OS) on your 4a5g, then (a) you've done a lot of careful debugging work, but (b) it sounds as if the most likely cause remaining is a hardware failure on your 4a5g, probably the cellular modem. If part of a device is going to break, it can often break when a device is power-cycled.

finitefutility Could it be possible a SIM is defective in just one phone?

In theory, yes.... maybe one of the contacts is scarred or corroded in exactly a way that matches a flaw in the phone's SIM contact array. But that would be a very unlikely event. Where I live, most carriers are willing to issue a replacement SIM for no cost or a minimal cost, so you might try that.

But I'm afraid the most likely situation is a hardware failure in your 4a5g.

            finitefutility Still no idea why a reboot would kill connection to carrier and why keeping the phone off for a few minutes restores it.
            finitefutility 3 power down attempts later, 2 reboots and no luck still. Currently the device is off, hoping it will magically come back somehow later. Very frustrating.

            This pattern -- an intermittent problem that gets worse over time -- is also consistent with hardware failure, especially hardware failure on an older device. Unfortunately, I honestly think all the evidence points in that direction.

                de0u If you have tried your SIM card in multiple phones, and it doesn't work in only your 4a5g, and you have tried a different SIM in your 4a5g but that doesn't work, and you have tried two different OSs (GrapheneOS and the stock Pixel OS) on your 4a5g, then (a) you've done a lot of careful debugging work, but (b) it sounds as if the most likely cause remaining is a hardware failure on your 4a5g, probably the cellular modem. If part of a device is going to break, it can often break when a device is power-cycled.

                Yes, it feels like it is a hardware failure.. which feels somehow "wrong" for it to happen at just 2.5 years after purchasing the phone but I guess it is what it is.