USB 3G Modem breaks IPv4 forwarding [17.01.2]

I am trying to use a 3G USB Modem as a second way to SSH into my Router.


config interface 'wan3G'
option device '/dev/ttyUSB0'
option apn 'internet.telekom'
option service 'umts'
option proto '3g'
option username 'congstar'
option password 'cs'
option ipv6 'auto'
option auto '0'

This Interface Works without a Problem.

But as soon as it becomes active, IPv4 for the regular ALN/WAN stops working, the router does not forward any requests.

This Interface is not associated with any forwarding Rule (LAN, WAN).

Only when the Router is restarted with this Interface disabled, everything works.
Even when Interface is enabled/disabled after startup IPv4 forwarding from LAN/WAN only works again after a router restart.

IPv6 works without a Problem.

Ok, solved it.

Just uncheck "Use default gateway" in Adapter settings.