Modern firmware numbers missing for mobile network connection

This doesn't make a lot of sense. I don't know what "modern firmware numbers" you would be talking about unless what you meant to say is that you lost your phone number – a different number supplied as part of the arrangement you have with your carrier (phone company).

I suggest contacting your carrier (phone company).

Again, the phrase "modern firmware numbers" makes no sense.

If you are referring to a mobile phone number – i.e., a phone number which someone could use to call you – you get that from a carrier (phone company) when you sign up for service with them.

"Firmware number" could refer to almost anything. The iOS version could be considered to be a firmware version number. There may also be additional settings that your carrier can push down to your phone – or have you pull down – in order to tailor your phone for use with their network.

Manually update your carrier settings on your iPhone or iPad - Apple Support

"When a carrier settings update is available, you'll be prompted to install it. Installation takes less than one minute, and you can keep using your device normally. If your carrier releases a mandatory update, you'll see an OK button instead of an Update button to let you know that the update was downloaded and installed."

But by and large, other than the iOS version, the versions of any other smaller pieces of firmware installed on the phone are an implementation detail.

You’re not going to be self-managing iPhone modem firmware.

Not past keeping iOS current, and installing carrier updates as available.

Maybe a device reset.

The USSD / MMI / short codes and associated feature availability can be carrier-specific and quite possibly iOS- or iPhone model-specific too, so if you’re reading something describing those codes as part of this modem firmware quest, you’ll want to check with the carrier about what they support.

TL;DR: Contact your cellular provider (your carrier). If the carrier cannot resolve the issue, contact Apple Support.

Do you mean modem firmware? Modern firmware makes no sense at all.