You can easily root the i535 with Towel Root apk. The i535 is not a good choice to try and use on AT&T network with only 2g being compatible.

 

Verizon network is CDMA and their phones have different radios for their network. AT&T network is GSM. While the i535 will receive 4glte on Verizon, it will only receive 2g signal on AT&T.

 

There is a YouTube video but I'm having trouble posting a link. It's how to activate a Verizon phone on AT&T.

 

Put in your AT&T Sim.
Go to settings, more,mobile networks,network mode. Change from LTE/CDMA to GSM/UTMS. Then go to APN settings and add settings for AT&T and save.

 

HSPA is 3g and the i535 doesd not have the correct hardware to receive 3g on AT&T network. You should be able to receive EDGE (2g). I have used my i535 with Straight Talk and their Verizon compatible sim which uses Verizon network. Never tried to run it on AT&T,2g internet speed would be to slow for me.
My best advice would be to take it to AT&T store and see if they can get it set up for you. Sorry I couldn't be more helpful.

 

I know people have had problems like this before with Verizon phones, both here on AF and seen it myself a couple of times when they've come to China. They're supposed to have been unlocked, but they won't accept anything but a Verizon SIM, "Not a Verizon SIM" or something like that. Even if though the phone itself does have the correct bands and modes for wherever it is. I've told them to go back to Verizon about it, as might not have been properly unlocked, or released from contract, or whatever, but that's rather difficult when you're in a foreign country.