GL-MT300 Ver 2 Subnet Mask

I can see the settings to change the DHCP range and IP etc. but there doesn’t; seem to be a way to change the subnet mask, unless I’m missing it.

Can I change this so that I can access the router settings from my main network?

Main router is on
Mango would be on

My basic networking education is very, very fossilized. Is this even possible to to do? If so what subnet mask should I use?