http://www.letmecheck.it/mtu-test.php

go here and click the Test it button, don't need to change anything
when it's done you'll have your MTU value

Then on your switch go to settings>internet>internet settings> choose your network from the list of wifi > change settings > scroll down to MTU and change it from the default 1400 to whatever you get from that test.

P.S. if the test site isn't working, just try setting the MTU to 1500.  Worst case scenario if something gets worse you can always set it back to the default of 1400.  No damage can come from this.