Adding 2nd router as guest AP

[#1]

I am trying to add a guest wifi AP using an old vodafone router. I have connected the 2 via ethernet cable and disabled the dhcp on the second router. The second router has a separate ssid and password. Everything connects fine on both ssid's but I get a security warning : 'this site can't provide a secure connection ' when trying to access websites. This happens on both ssid's.

[#2]

If you have connected the Vodafone router with an ethernet cable to it's WAN port, then you probably need to somehow disable IPv4 NAT, otherwise your Vodafone connected devices effectively become a network within a network because the router will be applying double NAT - which gives the symptoms you describe.

Alternatively, if the Vodafone router doesn't do some sort of network isolation between it's LAN ports and WiFi,  if you take the ethernet cable out of the WAN port and instead plug it into a spare LAN port, then quite often the LAN and WiFi then appear as interfaces on the same network as the other (broadband) router, because you are bypassing the Vodafone's routing function (i.e. the thing that is applying a second NAT).