So the new one I received two days ago and it was working out of the box, BUT with the 3g module instead of using the qmi mode and therefore using /dev/ttyUSB2 instead of /dev/cdc-wdm0. So I reconfigured the other one to the 3g mode and also using /dev/ttyUSB2 and it was working as well.
Then I switched the new one in the webinterface to 4g, then using qmi and /dev/cdc-wdm0 and it was still working. So I did the same to the other one and this is now working as well with that setup. I didn’t change anything else so could there be the possibility that something on the EC25 chip changed or is set/removed while switching from the 3g-module to qmi? Or is there a reset command for the chip that can be issued in qmi mode that could have caused this issue?
I only want to get to the root cause of this so that no one else is stuck at the same point 
Config set (out-of-box)
config interface ‘modem’
option proto ‘3g’
option ifname ‘3g-modem’
option device ‘/dev/ttyUSB2’
option country ‘country21’
option sp ‘isp2’
option dialnum ‘*99#’
option apn ‘internet.telekom’
option username ‘telekom’
option password ‘tm’
option metric ‘40’
Config now set in qmi mode
config interface ‘modem’
option ifname ‘wwan0’
option service ‘fdd-lte’
option proto ‘qmi’
option device ‘/dev/cdc-wdm0’