USG - setting MTU for Spark Fibre

I'm trying to configure the USG to use mtu 1500 for spark fibre.

pppoe mtu to be 1500

vif 10 mtu to be 1500

eth0 mtu to be 1508

Here is my config.gateway.json. It seems like valid JSON code but i am unable to connect to the internet with this config. Can someone help me figure out what is wrong with my config code

{ "interfaces": { "ethernet": { "eth0": { "vif": { "10": { "mtu": "1508", "pppoe": { "2": { "mtu": "1500" } } } } } }}}