On Tue, 16 Apr 2019 15:36:07 -0700, XPer
wrote:

My Win XP Home laptop has lost connectivity with my home LAN.
AT&T installed a replacement modem. Other devices made the proper hookup.

My laptop says it is connected and say it is an excellent connection but
it will not connect to the internet or to my Buffalo NAS that my other
desktop windows PC connects to without any hesitation.

Browsers do not work. Chrome say it cannot connect. As does Firefox.

Strangely my Buffalo NAS app finds the NAS but says it probably will not
work as it has a limited connectivity.

What does that mean ?

Please offer steps to troubleshoot.

I would start with a "Repair connection".
Start/Settings/Network/LAN Connection/Support
If no joy after that power everything off and back on and see how you
do.