Hi,
glad to hear it’s working 
If you don’t want to discard the DHCP DNS altogether, it seems there’s also a setting ipv4.dns-priority that might help in fixing the order of your nameservers.
Maybe try setting that on both connections, with the cellular having a lower value; note that the default values are 50 for VPN and 100 for others and 0 selects the default.
Based on your routing table, it should then first try the 172.30.8.5 and 172.30.8.6 via cdc-wdm0, only going to 192.168.2.10 via eth0