When trying to open a link from Outlook Mobile on my iPhone (a.k.a. SafeLinks) when connected to my Deco net, I always get the error:
 

Safari can't open the page because it cold not establish a secure connection to the server. 

I can't say when the issue occurred, but I'' pretty sure it worked in the past with an older firmware. 
I've also changed internet provider, but since the links do work when bypassing the Deco WiFi router, I've isolated the issue to the Deco device itself. 
 

I have seen similar issues in this forum that was solved by using a cloned MAC-address. But I haven't resorted to trying that solution yet.