@ gazaai
I did what you say about hub one as the wifi only but sadly it didn't work. Do I have to make sure billion is on bridge mode? If so, then I cannot touch it at the moment because DLM is on it for a moment. So, I best leave Billion modem alone.
@adslmax
It definitely works I've used several HomeHub 4,5s and one PlusNet on my network as access points with my Billion.
No you don't put the Billion in bridge mode as we still want to use the Billion as your router.
Are you sure you connected the Plusnet to the Billion with an Ethernet from a
LAN port on both devices. Not a WAN port.
Also if there is Smart Setup on the plus net disable it.
Your screenshot of the DHCP looks fine, just double check there is nothing on your network that uses 192.168.1.253 already as that would cause an issue as well.
Also I assume you are using the Billions default DHCP range and its IP is 192.168.1.254?