I have two of the same router both connected over ethernet. 

The first is connected to the landline adsl connection and set on "DSL Modem Router Mode"

The second is also set on "DSL Modem Router Mode" and DHCP disabled. 

The main SSID works fine with no issues. (gave it the same ssid on both)

Today i tried to activate the guest network on both routers (gave it the same ssid on both) but some devices couldn't connect to it and others connected but with no internet. 

What i tried: 

1- eanabling DHCP relay on the second router. (didn't work, maybe worked once)
2- connecting devices to the main router's (the first one) guest network. (worked)
3- connecting the same device back to the second router's guest network (didn't work)