Sounds like the fix is for 5 years only.
The SMR is probably the standard variable rate and currently 3.99%, in five years time it could be much higher than 3.99%, you will have to pay whatever it is at the time, although you will be free to find another deal else where or with the same lender. If you cant find another deal then you will pay the going SMR rate until the end of your mortgage and the rate will vary over that period.