Hi EddieP,
I have checked your code and is working correctly. You should change the below line of code
from
url: '<%=ResolveUrl("~/CS.aspx/GetCustomers") %>',
url: '<%=ResolveUrl("~/CS.aspx/GetLink") %>',
to
url: '<%=ResolveUrl("~/2019.aspx/GetCustomers") %>',
url: '<%=ResolveUrl("~/2019.aspx/GetLink") %>',
As your partial class is _2019. Except that all code is correct.