Hi,
This is a known issue. You can refer to the following thread for more details:
On firmware 4.7.13, the workaround is to edit:
/lib/netifd/proto/qcm.sh
Comment out the line:
pdp_type='-4 -6'
Then restart the network service:
/etc/init.d/network restart
Alternatively, you can try a newer beta firmware version, such as 4.8 or 4.9, where this issue should already be addressed.