Connecting xFinity Modem to personal Router

Hi,

I am constantly facing issues with my xfinity connection. The speed sort of drop to 0 multiple times within a min and it's really frustrating. We have tried everything (called up the tech specialists about 3-4 time) and nothing seems to be working. Last thing we want to try is using our personal router and only limited the xfinity capabilities to a modem as we have seem with the ethernet cable the network seems to be a bit more healthy. I have a tp-link 1750 router. But I am unable to connect this to the modem. Has anyone in the past tried this process and can guide me or share steps, that'll be great. 

Steps we have tried:

- Disabled the wifi capabilities of the xfinity modem from the admin portal

- Restrating the modem

- Resetting the personal router

Even after all this, router is not able to connect with the modem through the ethernet cable (I am plugging it in port 1), but when I use the same port with laptop it works. Not sure what other things we need to take care of.