How to properly setup a reliable roaming WiFi with Airport Base Stations and an existing router
First, you will not receive adequate routing performance on network by way of the ISP's embedded router. 

How to build WiFi 101

Log in to the AirPort devices and configure:


Internet set to DHCP
the Network to Bridge mode
the Wireless to create a wireless network 
Enable an alternate name on the 5 GHz network (you could simply append "(5 GHz)" to the wireless network name)
leave the wireless channels set to automatic
Change Radio mode to 802.11a/n - 802.11b/g
Change the country to your own to avoid interconnection problems on 5GHz


Place the first AP at centrally in one extremity of the building. Join the AP on the 5 GHz network. Option-Click the WiFi menu to read the RSSI value. Determine the next AP location by by measuring where the signal dips below 65-70. 

Once you've placed all the APs join all 5 GHz capable WiFI devices to the 5 GHz WiFi. Join all remaining devices to the non-5 GHz (2.4 GHz) network. 

Protip: There's a great tool called WiFi Diagnostics in /System/Library/CoreServices. Check it out.