Presumably you are not getting this message if you access websites using a tablet or computer so it's something to do with your mobile? Have you set up the time and date on your mobile correctly Settings->Date and time -> Automatic date and time. I'd probably restart my phone too. You never know, it may work. If it doesn't, try Googling 'security certificate not trusted', adding mobile phone make and model and operating system version and the web site you are accessing when you get this warning. If you don't get anywhere with this, put the same information on here, I expect there is a Gransnetter that would be able to help, given enough information.
