Whitelisting URLs on my Gateway

Hi! I have a smart wifi scale. It works perfectly if I have my own router. It never works on company-provided modem/router combo devices. The info I was given by the manufacturer is that I need to whitelist 2 URLs on my gateway to set it up then I believe the scale will talk only to my phone over my local network. How can I do that? I see no such settings in the TMo internet app, FAQs, etc. Almost all of the whitelisting posts deal on this forum are issues with VPNs, email-to-text or sending business-related SMS messages. I just want my scale to work again. Thanks!