I recommend reserving the IP address through your two routers rather than using a static IP address. Letting the router manage the IP address would have made transitioning between the two routers that much more easier . . .
That sounds great, but I can't find anything about reserving local IP addresses in the manual of the R90 router. The Netgear router that I use at home, have that option and I have reserved a lot of LAN IP addresses there, laptops, a D-Link NAS-box and the D-Link camera.