How to change a routers external IP address

> So [..]

   Never begin a discussion with "so"; like "therefore", it requires an
antecedent.

> [...] I have multiple routers in one home network [...]

   Why?  That's probably your primary mistake.  Cascading multiple
routers can cause multiple problems, including ones like yours.  Are all
these routers model R6900?

   Unless you have some very good reason to use multiple
routers/subnets, the likely solution is to configure one router as your
main/only router, and the others as wireless access points.  This should
put all your devices on one big (extended) LAN, with all the port
forwarding done by the main router.

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual.  Look for "Use the Router as a
WiFi Access Point" (assuming that it's an R6900).