After many headaches, I found out the issue.
I entered this in SSH mode:
gl_modem -B 0001:01:00.0 SAT sp "AT+QSCAN=3,1"
The response:
+QSCAN: "NR5G",262,01,154570,478,-97,-11,30,0,4F3104701,802A,52,28,15,2,-
+QSCAN: "NR5G",262,01,431070,421,-88,-11,38,0,4F328B101,802A,106,1,17,2,-
+QSCAN: "LTE",262,03,1600,242,-,-,-,0,17C3019,C954,-,3
After looking up the 262 MCC and MNC 01 at mcc-mnc.com (type in the search field: "262, 01"),
I found out that the cell tower did not have O2 as a provider; it only had Telekom. That is why it threw a SIM card not registered error.