Hi markbsac,
A warm welcome and thanks for posting on our community forums. We're sorry to hear that the adaptor you have been sent does not fit the cable which supplies your router.
This is a standard adaptor which we send to all customers who have had the home phone switch over so you should not encounter any issues with this.
As our valued member @goslow has posted clear instructions for the adaptor, can you make sure this seated in the TEL1 port.
Even though you do not use your landline on a frequent basis, as @goslow has advised, we would recommend every so often due to the above mentioned.
If you have any further queries or questions about this, please feel free to post back.
Kind regards Jodi.