Route all traffic over Fiber VLAN
Solution 1:
If you're using a bridge interface, you should assign an ip address on it, not on a bridge port. In your case your settings will look like:
auto enp5s0
iface enp5s0 inet manual
auto enp5s0.300
iface enp5s0.300 inet manual
auto br0
iface br0 inet dhcp
bridge_ports enp5s0.300
bridge_stp off