Model: Archer C2600

Hardware Version: V1

Firmware Version: 1.1.8 Build 20170306 Rel. 44838

Hi I recieved a new fibermodem after the old one broke. The old one didn't had a router so I bought one, now my modem has been replaced it has a fiber cable with goes into the required router delivered by my ISP that I'm required to use because of TV. I still wanted to use my TP-Link router because of features one of them is openvpn. I already notice that the internet ip is the local ip that my isp router has assigned to it. I think that is the reason why my openvpn doesn't work anymore. I also cant portforward anymore. I portforward my TPLink and my isp router. I tried in my isp router both the ip assigned to the router and tplink router ip (there are both in different pools that I cant change because of lan ip and ip pool cant be in the same subnet it says ). Cant I use the tp router with portforward and do I need to buy a raspberry pi or something and connect it to my isp router?