What you have is a router. What you need is a combined modem/router. You have 3 options:
1. Buy a separate modem to go with your Asus router. I would recommend a model 3b Huawei HG 612 from an auction site such as Ebay if you are on a fibre connection. If you are on ADSL I have no recommendation..
2. Turn your Asus router into an Access Point and connect it to your Plusnet hub. If you do a Google search you will find the simple instructions. You then connect this to your existing Plusnet hub by connecting a LAN socket on each together using an Ethernet cable. You can then go into your Plusnet hub advanced wirelwss settings and switch off the hub wireless so that it won't potentially affect the Asus wireless
3. Forget both devices ans buy a BT Smarthub 6 from an auction site. This is a combined modem and router like you have now. They cost very little money and have better wireless than any Plusnet hub. They need special set up to work with Plusnet and you will find the simple instructions at the top of this Router board. Many of us use them and they work fine.