Why is the Wi-Fi connection so slow on my S6 Edge.

[AP-Hinds]

Why is the Wi-Fi connection so slow on my S6 Edge. DN SET app download fixed it but I was unable to send text while it was running.  Why would I need to download an app that isn't from Samsung to get my wi-fi connection to work?  When I first got my S6 the wi-fi worked fine and only in the last several weeks has it changed.  It isn't modem or router specific as it is slow on every wi-fi connection I try.  Other devices in my home are working perfectly normal and I even went so far as to have my modem/router replace by my ISP.  I have performed system restore and or reboots of both the phone and modem without change.  What is the correct fix?