S-6 / Marshmellow / WIFI dragging

Hi - bought very nice S6 from original owner (AT&T) - cleaned, and professionally un-locked.
Put Cricket Sim in - all works excellent except WIFI.....either very very slow or no connect at all.
Can use maps and google and such with WIFI off, and speed is excellent.
Turn on WIFI and connections suck.
My WIFI is excellent and reads VERY STRONG in WIFI setup....

Is there any left over AT&T stuff that is sucking WIFI to the ground?

Thanks for any ideas....

/Don

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S-6 / Marshmellow / WIFI dragging