I recommend that you bridge the ISP modem and use the extra router as your main host router. Connect Computer 1 and NAS to the main host router. This will eliminate any double NAT conditions that cause problematic connections with having 2 routers on the same line.
If the main host router does not have additional LAN ports, install a external Gb network switch to provide you with extra LAN ports. This also helps eliminate any un-necessary router processing that is not needed for large file xfers between computers and NAS.
Might look like this:
ISP MODEM(Bridged)>Main Host Router><Network switch<>NAS and PCs connected to switch.
>For any game consoles, connect directly to the routers LAN ports and not the network switch.