OP
Octopus
It's not just about joining 2 routers together, you need to setup SSID, DHCP, SECURITY & IP correctly.
Main router:
I.P Address - 192.168.1.1
Subnet mask - 255.255.255.0
DHCP - on
2nd router (Access point):
SSID - The same as your main router
Security authentication method the same as your main router
Security encryption type the same as your main router
Security key the same as your main router
I.P Address - 192.168.1.2
Subnet mask - 255.255.255.0
DHCP - off
Make sure you only have 1 DHCP server running on the network as there will be I.P conflicts.
The Devolo ones I use, you plug in near the router, push a button on the router and then the Devolo, then the Devolo uses all the settings per the router. easy peasy