Arduino UNO + SIM808 GSM shield , +CME error: operation not allowed at AT+CSTT, AT+CIICR and AT+CIPSEND

Question

I'm pretty new to programming and hope that someone knows what's the cause of the issue. I'm building a remote IoT weather station for vineyards which should send the sensor data to a python tcp socketserver running at home. Most of the code (arduino sketch and python server) were found on github (https://github.com/lvzon/weatherstation). As written in the title, some errors occur. After setting AT+CMEE=2 the more detailed "+CME error: operation not allowed" comes up. I already tried some solutions that worked on others, like adding an external power source to deliver 2 amps when needed or trying different Sim cards at different locations (in Austria). Here's the verbose output:

16:45:01.858 -> Wind speed: 0.0 Pulses: 0 Time (ms): 172800  Dir: NE  Value: 1
16:45:14.397 -> Temperature Celsius: 4.92 Humididy %: 79.81
16:45:14.444 -> Pressure (mbar): 1001.58
16:45:14.492 -> Soil temperature Celsius: 5.06
16:45:14.492 -> Soil moisture reading: 479
16:45:14.539 -> Current (A): -5.10
16:45:14.539 -> Rainfall: 0.00 Pulses: 0 Time (ms): 172800
16:45:14.585 -> 948916800,172800,4.92,4.92,4.92,79.81,79.81,79.81,0.00,0.00,0.00,0.00,0.00,45,NA,-5.10,260,1001.58,5.06,479
16:45:14.726 -> ATI
16:45:16.650 -> ATI

16:45:16.650 -> SIM808 R14.18
16:45:16.697 -> 
16:45:16.697 -> OK
16:45:16.697 -> AT&V
16:45:46.675 -> AT&V

16:45:46.675 -> DEFAULT PROFILE
16:45:46.721 -> S0: 0
16:45:46.721 -> S3: 13
16:45:46.721 -> S4: 10
16:45:46.721 -> S5: 8
16:45:46.721 -> S6: 2
16:45:46.769 -> S7AT+COPS?
16:45:47.711 -> AT+COPS?

16:45:47.711 -> +COPS: 0,0,"one"
16:45:47.759 -> 
16:45:47.759 -> OK
16:45:47.759 -> AT+COPS=?
16:46:07.726 -> AT+COPS=?

16:46:07.726 -> +COPS: (1,"A1","A1","23201"),(1,"T-Mobile A","TMO AAT+CSQ
16:46:08.759 -> AT+CSQ

16:46:08.759 -> +CSQ: 19,0
16:46:08.759 -> 
16:46:08.759 -> OK
16:46:08.759 -> AT+CREG?
16:46:09.745 -> AT+CREG?

16:46:09.792 -> +CREG: 2,5,"0BD6","5578"
16:46:09.792 -> 
16:46:09.792 -> OK
16:46:09.792 -> AT+CMEE=2
16:46:10.262 -> AT+CMEE=2

16:46:10.262 -> OK
16:46:10.309 -> AT+CFUN=1
16:46:10.778 -> AT+CFUN=1

16:46:10.778 -> OK
16:46:10.778 -> AT+CGATT=1
16:46:12.798 -> AT+CGATT=1

16:46:12.798 -> OK
16:46:12.798 -> AT+CREG=2
16:46:13.315 -> AT+CREG=2

16:46:13.315 -> OK
16:46:13.315 -> AT+CSTT="TM","",""
16:46:17.354 -> AT+CSTT="TM","",""

16:46:17.354 -> +CME ERROR: operation not allowed
16:46:17.400 -> AT+CSTT?
16:46:27.359 -> AT+CSTT?

16:46:27.359 -> +CSTT: "TM","",""
16:46:27.406 -> 
16:46:27.406 -> OK
16:46:27.406 -> AT+CSTT=?
16:46:37.354 -> AT+CSTT=?

16:46:37.401 -> +CSTT: "APN","USER","PWD"
16:46:37.401 -> 
16:46:37.401 -> OK
16:46:37.401 -> AT+CIICR
16:46:42.385 -> AT+CIICR

16:46:42.385 -> +CME ERROR: operation not allowed
16:46:42.432 -> AT+CIFSR
16:46:44.404 -> AT+CIFSR

16:46:44.404 -> 10.6.155.157
16:46:44.404 -> AT+CIPSPRT=0
16:46:45.438 -> AT+CIPSPRT=0

16:46:45.438 -> OK
16:46:45.438 -> AT+CIPSTART="tcp","89.144.xxx.xx","80"
16:46:53.443 -> AT+CIPSTART="tcp","89.144.xxx.xx","80"

16:46:53.490 -> OK
16:46:53.490 -> AT+CIPSEND=?
16:46:53.960 -> AT+CIPSEND=?

16:46:54.008 -> +CIPSEND: <length>
16:46:54.008 -> 
16:46:54.008 -> OK
16:46:54.008 -> AT+CIPSEND?
16:46:54.479 -> AT+CIPSEND?

16:46:54.527 -> +CIPSEND: 0
16:46:54.527 -> 
16:46:54.527 -> OK
16:46:54.527 -> AT+CIPQSEND=1
16:46:54.998 -> AT+CIPQSEND=1

16:46:55.045 -> OK
16:46:55.045 -> AT+CIPSEND
16:46:59.012 -> AT+CIPSEND

16:46:59.059 -> +CME ERROR: operation not allowed
16:47:29.187 -> AT+CIPCLOSE

As you can see I tried to get more information, that's why the output is a little bit long and messy. The quality of the signal is ok and the providers shown after "AT+COPS=?" are all supported. On the Things Mobile (provider) website I can see that the gsm module connects to the network and sometimes sends a few kb (1 to 80) of data (maybe that's just caused from connecting to the network?), but nothing ever gets to the tcp. The Sim808 shield for the arduino uno (r3) was bought from elecrow.com and is used with an external gsm antenna. That specific module normally works well in Europe, at least that's what I've heard. I'd be very thankful if someone could help me to find a solution. Thanks in advance!