App Subscription Question (without IAP)

Hello,

My client has a security system app that will be going in the store soon. They are worried that the app will be rejected as they plan on promoting the external subscription signup (which also uses a separate payment system btw). 

Are we allowed to talk about / link to the monthly subscription for a security plan per the guidelines? 

Will this be seen as circumventing the 30% fee? 

Does app functionality changes based on the subscription signup impact rejection?

What else could I be missing?

Thank you,

Evan