LTE works, GSM does not

[NaN INF NaN]

I'm enjoying SXMO on my new PinePhone (PPHONE-BE2). For the most part, it is working well. When I have LTE reception, calls and data work without problems. However, LTE coverage is still patchy in Germany, UMTS is disabled and 5G is unsupported on PinePhone. So whenever LTE is unavailable, the phone should use GSM and that is not working at all. I have never seen the phone registering successfully in the GSM network of my carrier. Same with the preinstalled Plasma Mobile (in Manjaro ARM I think).

Here is some data that may help with diagnosis.
* Carrier: Congstar (Germany, a brand of Telekom).
* The SIM is old (from around 2008) but GSM works in another phone.
* I flashed an open source firmware onto the modem but had the same problem before doing so.
* Below is the output of mmcli -m 0 and mmcli -m 0 --3gpp-scan --timeout 300 in a spot where I do not have LTE coverage.
* Note that 3gpp-scan finds the GSM network of O2 but not the one of my carrier.
* It also finds the LTE network of my carrier (26201 - TDG) but I think reception is too bad to connect.

Any thoughts on how to fix this? I consider ordering a new SIM. I may also switch to a different carrier, so recommendations for German carriers that work well with PinePhone would be appreciated.

Code:

  -----------------------------------
  General  |                    path: /org/freedesktop/ModemManager1/Modem/0
           |               device id: 9717313523c02e0afbec903cac5df14f8bd7e778
  -----------------------------------
  Hardware |            manufacturer: QUALCOMM INCORPORATED
           |                   model: QUECTEL Mobile Broadband Module
           |       firmware revision: EG25GGBR07A08M2G
           |          carrier config: Commercial-DT
           | carrier config revision: 05011F1C
           |            h/w revision: 10000
           |               supported: gsm-umts, lte
           |                 current: gsm-umts, lte
           |            equipment id: 860195053807878
  -----------------------------------
  System   |                  device: /sys/devices/platform/soc/1c1b000.usb/usb2/2-1
           |                 drivers: qmi_wwan, option
           |                  plugin: quectel
           |            primary port: cdc-wdm0
           |                   ports: cdc-wdm0 (qmi), ttyUSB0 (qcdm), ttyUSB1 (gps),
           |                          ttyUSB2 (at), wwan0 (net)
  -----------------------------------
  Numbers  |                     own: 49<redacted>
  -----------------------------------
  Status   |                    lock: sim-pin2
           |          unlock retries: sim-pin (3), sim-puk (10), sim-pin2 (3), sim-puk2 (10)
           |                   state: enabled
           |             power state: on
           |          signal quality: 42% (cached)
  -----------------------------------
  Modes    |               supported: allowed: 2g; preferred: none
           |                          allowed: 3g; preferred: none
           |                          allowed: 4g; preferred: none
           |                          allowed: 2g, 3g; preferred: 3g
           |                          allowed: 2g, 3g; preferred: 2g
           |                          allowed: 2g, 4g; preferred: 4g
           |                          allowed: 2g, 4g; preferred: 2g
           |                          allowed: 3g, 4g; preferred: 4g
           |                          allowed: 3g, 4g; preferred: 3g
           |                          allowed: 2g, 3g, 4g; preferred: 4g
           |                          allowed: 2g, 3g, 4g; preferred: 3g
           |                          allowed: 2g, 3g, 4g; preferred: 2g
           |                 current: allowed: 2g, 4g; preferred: 4g
  -----------------------------------
  Bands    |               supported: egsm, dcs, pcs, g850, utran-1, utran-4, utran-6, utran-5,
           |                          utran-8, utran-2, eutran-1, eutran-2, eutran-3, eutran-4, eutran-5,
           |                          eutran-7, eutran-8, eutran-12, eutran-13, eutran-18, eutran-19,
           |                          eutran-20, eutran-25, eutran-26, eutran-28, eutran-38, eutran-39,
           |                          eutran-40, eutran-41, utran-19
           |                 current: egsm, dcs, pcs, g850, utran-1, utran-4, utran-6, utran-5,
           |                          utran-8, utran-2, eutran-1, eutran-2, eutran-3, eutran-4, eutran-5,
           |                          eutran-7, eutran-8, eutran-12, eutran-13, eutran-18, eutran-19,
           |                          eutran-20, eutran-25, eutran-26, eutran-28, eutran-38, eutran-39,
           |                          eutran-40, eutran-41, utran-19
  -----------------------------------
  IP       |               supported: ipv4, ipv6, ipv4v6
  -----------------------------------
  3GPP     |                    imei: 860195053807878
           |           enabled locks: sim, fixed-dialing
           |             operator id: 26203
           |            registration: idle
  -----------------------------------
  3GPP EPS |    ue mode of operation: csps-1
           |  initial bearer ip type: ipv4v6
  -----------------------------------
  SIM      |        primary sim path: /org/freedesktop/ModemManager1/SIM/0
           |          sim slot paths: slot 1: /org/freedesktop/ModemManager1/SIM/0 (active)
           |                          slot 2: none

Code:

  ---------------------
  3GPP scan | networks: 26203 - o2 - de (gsm, current)
            |           26203 - o2 - de (lte, forbidden)
            |           26202 - Vodafone (lte, forbidden)
            |           26201 - TDG (lte, available)