I have a spare modem wifi router that I have connected via Ethernet cable to my cable wifi router, in order to extend the wifi signal into a different part of the flat.
I disabled dchp on the second router. Strangely the Internet comes through fine on the second one, but no, or very little data is coming through on my first router (neither wifi nor wired). Windows doesn't report any problems with yellow triangles.
Do I need to change any other configuration?
To setup a second router as an access point, attach it to the main router LAN port to LAN port (as it sounds you have), disable DHCP on the second (again you've done this), assign the AP a static address outside the DHCP address range allowed to the first router for assignment, use the same SSID, use different radio channels on the AP, use the same WPA/2 key.