Well it's not too surprising when you have two routers connected to each other.
You've probably created a second NATted subnet unintentionally.
Set the Tenda to Access Point mode, if it has that. Disable DHCP on it. And connect it to the other router via one of its LAN ports, NOT the WAN port.
Oh, and make sure the Tenda's own IP address is set to something different from the Westell's (but on the same subnet).