ZTE MF281 Cannot make LTE modem work

Well, then you can try

AT+CGDCONT=1,"IP","internet.telekom"
AT+CGACT=1,1

Ideally we need to see the connection log from the device running OEM firmware.

P.S. just to double-check:

AT+ZGDCONT=?;+ZGDCONT?
AT+ZGACT=?;+ZGACT?

hmmm.

AT+CGDCONT=1,"IP","internet.telekom"

OK
AT+CGACT=1,1

+CME ERROR: unknown error
AT+ZGACT=1,1


ERROR

First Config instruction is OK, 2nd returns error.
Same with Z takes a while and returns then error.

and

AT+ZGDCONT=?;+ZGDCONT?

+ZGDCONT: (0-15),"IP","APN NAME"

+ZGDCONT: (0-15),"IPV6","APN NAME"

+ZGDCONT: (0-15),"IPV4V6","APN NAME"

OK


AT+ZGACT=?;+ZGACT?

+ZGACT: (0,1)

OK

Seem like some instruction are read-only or have another set of parameters to write.
What a weird device

Then you can try

AT+ZGDCONT=1,"IP","internet.telekom"
AT+ZGACT=1,1

Wasn't this the original commands ?
Tried:

AT+ZGDCONT=1,"IP","internet.telekom"

+CME ERROR: Invalid Param
AT+ZGACT=1,1


ERROR

The original command was longer: AT+ZGDCONT=1,"IP","internet.telekom","",0,0
The last one perfectly fits into the syntax shown by the modem:

AT+ZGDCONT=1,"IP","internet.telekom"
+ZGDCONT: (0-15),"IP","APN NAME"

I don't understand what it is complaining about.

Me neither ... no idea.

Many many thanks for your input and our long debugging session.

I am going to investigate a HW issue in the area of the sim card slot now
and disassemble the device again.
The serial port which I accessed to flash openWRT is next to it.

Best regards,
funcarver72