So, in the beginning I have this 6a with Graphene with a physical SIM, say X.
Then I get another physical SIM Y. I upgrade X to an eSIM, and insert Y in the physical SIM slot.
X goes out of service for a while due to the upgrade procedure taking some time but the
eSIM activation works really well and SIM X as an eSIM now, is setup and working within the day.

Y however, always shows as: No Service. I am able to view the SIM information and the
number etc. Just that no services are available for Y, on this device.

I later swapped the SIM Y with another physical SIM Z from a perfectly fine Samsung device.
Services for SIM Y are active at once on this Samsung devices and the SIM Z in the physical Slot now
shows as No Servics. Just like Y.

I have rebooted multiple times, enabled/disabled DSDS multiple times.
Reset the network settings. Unsure of what else I can do to get more insight into the problem.
Device is running the latest OS stable release.

Let me know if you need any more information to debug this issue
and what further steps I can take to debug this issue. Thanks in advance.

Have you tried manually adding the APN for the mobile network of SIM Y?

What the heck, why didn't that occur to me lol : )
Anyways that worked like magic. Thanks a lot.