limbot
5
Occams razor 
If you are getting an IP address in a different subnet (I’m assuming you’re getting 192.168.8.X subnet on the Mango side?), then extender mode is not being setup properly.
In extender mode, the Mango should get an IP address from your primary router via DHCP and as such be in the same IP subnet. You can of course use a static IP on your primary router to set a static IP .
If you attach to either the LAN or WAN port of the Mango (the WAN will be a LAN port in extender mode), then if it’s using DHCP it should pick up an IP in your primary router subnet.