Factory Unlocked Firmware on Verizon Device

I recently got a Galaxy S6 from a friend that was broken. They said if I could fix it I could have it. After replacing the display and digitizer I began setting up my phone for use with AT&T on the Unlocked device. I put the AT&T APN configuration in but I'm still seeming to get H at times when I should be on 4G. In some places I have full bars on 4G but when the signal strength drops even a little bit I seem to lose this instantly. I was wondering if it's possible to flash the stock...

Factory Unlocked Firmware on Verizon Device

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