I, too, have service from an MVNO that uses T-mobile’s network, and the command always shows T-mobile as the LTE signal for me. Could yours be showing something related to your network signal enhancer?
Otherwise, the displayed MCC+MNC code might help you identify the source of your signal:
(P.S. I know for a fact that there are other strong LTE signals in my area besides T-mobile.)
Is there a reason I cannot access that website from my home wifi network? I am getting an “not secure” error. I am assuming I have a firewall of some kind.
Never mind I just used a Tor browser.
Can you elaborate on what this is? e.g. make/model
If you are interested in the cellular network signal strength then the magic AT command is
AT+CSQ
The response will be
+CSQ:n,m
where n is the number you are interested in.
n will either be 99 (unknown) or it will be a number in the range 0 to 31 where 0 is weakest and 31 is strongest. (Can be converted arithmetically to a dBm value but that conversion may hinder rather than help here so details omitted.)
Tips & Tricks · Wiki · Librem5 / Librem 5 Community Wiki · GitLab
I imagine that mmcli -m any | grep 'signal quality' is giving you the same information except that
mmcli is converting the actual number from the modem into some other numeric representationmmcli may sometimes give you a cached value (which probably doesn’t suit the purpose).Is that Linuxy enough? 
This is very Linuxy. I will try some of what you suggest and report back.
Ok. Your tool was very helpful. I improved the cellular service at home. Not perfect, but an improvement.
One odd problem I now have is I get no ring tone when I call someone. It is very disconcerting to not hear a ring tone when calling.
Anybody else have no ring tone when calling? Anybody solve this?
Previously reported: Report: No ringing on my end when placing outbound calls, but the call goes through
Crimson fix? Very oddly, I get a ring tone when I call using JMP.chat. Odd.
I am still playing with my poor at-home cellular coverage. I have some home coverage now but another odd issue persists: my cellular modem keeps picking up a different signal and (I believe), because it is stronger than my Mint/T-Mobile Signal, it switches to the stronger signal, consistantly. Very annoying.
Is there a way to point the modem at only a particular “signal” - e.g.: the signal I want it to recognize and use?
Are you still able to make a call or does the phone deregister? I’m asking because I’m having this issue where in some places in my area it’s not registering but I get sometimes full bars. It’s very odd, and the reason I’m sending my phone back again to Purism.
I don’t think the command would show anything but the mobile network that the modem is connected to, and it can only connect to the network that corresponds to your SIM card.
In fact, you should see your phone number IMEI displayed in the output.
Which MCC and MNC designators are showing in the output?
My money is still on the signal that your extender is emitting, as I mentioned earlier.
Are you asking for that information when I am connected to the Mint network or without connection or both?
When you see the unidentified LTE signal.
T-mobile/Mint/etc. should be something like 310 and 260 (or 240, etc.).
I don’t know anything about extenders, but isn’t it acting sort of like a bridge or access point, except instead of WiFi, it’s the mobile network…? When you’re connecting to the extender, it might show something other than Mint/T-mobile.
Could your extender be a clandestine surveillance device produced and controlled by a foreign nation? 
I can make a call when the phone says I am connected to Mint. Not on the other signal.
Thank you for all your help. What command output do you mean? I don’t see a MCC or MNC designation using mmcli --modem=X
I knew it. I just needed someone to confirm it.
Try mmcli -m any and look for a section toward the bottom that contains those.
And I was mistaken about your phone number being displayed… That’s in Android. The command output does show your IMEI, though.
Look for “operator id” and “operator name.”
Yep, 310260 when connected to Mint. On the money.
The phantom signal reports 310410.
Can I tell the modem to only recognize 310260?
Sure! With the APN setting.
Going to the MCC MNC list website, sorting on MCC, then searching “410” reveals that it’s AT&T.
Is the phantom signal still there if you unplug or turn off the extender?
Does Mint roam on AT&T when there’s weak T-mobile signal? (doubtful)
Has the manufacturer of your extender partnered with AT&T to provide range?
Was the extender provided by T-mobile?
This is getting exciting. Progress is being made here…
I have tried this. I apparently am not skilled enough. How exactly to I make the modem only recognize Mint/310/260 only?
As I type this, my modem has probably jumped from one signal to the next 3-5 times. Sometimes it goes right back to Mint, sometimes it doesn’t. Drives me nuts.