help. guys . can not connect internet

Hi All

i hope you guys enjoying the weekend

i have a question about connecting internet here

i got an iphone 6s(ios 10.0.2) from my friend who used to use it for ATNT but it is unlocked

but i am using Ultra Mobile which is used by T-mobile network so i put it in and checked the signal

like ULTRA and LTE on the screen, yes it was shown and calling, texting and using i-message were ok by data

i just turned it off and left it for 1-2 hours (could not check the internet. no time )

when i got back home and turned it on , checked and downloaded some apps by data, not wifi

it was not working and i searched online for using internet so i went to ultralmobile.com and set the APN for iphones

turned it off and on it back ... still was working ..but i did it several times APN setting , turning off and on.

if it is working... but my hope was so high ...

i went to a ultra mobile shop and a guy told me that the setting is ok , he did not know why it was not working

he called customer service center and spoke with someone and finally he told me that use it after 2 hours

so i waited for 3-4 hours at home but was not working

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additional info :

when i reset all and turn it on then i can calling, texting and using i-message but not internet

when i set the APN of ultra mobile for iphones, then i can calling, texting but not intenet and i-message

is there any way to resolve this issue? should i go to apple store ?

(it is still under warranty by 11/20/2016)

thank you guys