Sounds like you found a limitation of the Network Extender.
Network Extenders are not full blown cellular towers like the ones outside your home. They only provide the minimum number of the channels and frequencies necessary to operate your End User Devices. Due to that limited functionality I'd guess that whatever is driving eSIM on your iPhone breaks/does not have the right communication to work.
Both of your phone numbers can make and receive voice calls and send and receive SMS and MMS.2 Your iPhone can use one cellular data network at a time.
It might be worth checking in with Apple to see if they have any limitations with this technology and home network extenders. Id guess they have the same limitations with other Service Providers and home network extender devices too.