The certificate for the m.48months.com site expired last week. Somebody else had the same problem here a few days ago and I reported the problem to 48 support but it obviously still hasn't been fixed. Try topping up some other way - e.g. in the full www.48months.com site rather than via the app or mobile site if they're having this problem. You may need to use a computer/laptop if your phone insists on going to the mobile site.
Hope this helps.
I am not a 48 employee so if you need to contact 48 support then use option 3 or the chat facility here.