I had the same issue previously, but 1528 did not do
I have since changed build on that unit and can't remember the actual
value I used, But it was larger.
All I did was increase the value until the warning disappeared, and
everything started working as advertised.
what do you mean with "working as advertised" ? It should work even if you get
this warning. All it does is fragmenting your traffic.
Setting the MTU to 1528 works for me. Without providing more information about
your setup and giving us the number which seemingly "fixed" it for you nobody
can help you.