When I boot android device having a SIM on roaming it took around 2-10 minutes to get cellular reception. I've tested this on multiple devices, similar behavior.
One interesting thing is, if device is already booted and then I inserted the SIM, it instantly gets cellular reception.
For a non-roaming SIM it works instantly even after boot.
Edit: The SIM provider told us it's an eSIM, though it looks like a regular SIM card.
If you've any explanation on this, could you please share?