Hello,
We have a hybrid development where our Skype for Business 2015 On-Premise server is having some users have trouble signing in thru Iphones, Ipad, and a windows 10 laptop. Laptops before had issues, but all I did was add them on the domain and then they started to work. But this laptop is a personal one for the user. I check remote connectivity test and everything is green. We can't seem to get these mobiles to connect. The error that they pop is can't connect to server right now try again later.