No VPN will hold more than 1 minute

it is very strange i replaced my ATT iphone 4s with a sprint iphone 4s bc i changed carrier.

now my VPN service that worked fine with my previous phone won't stay connected for more than a minute or two.

i contacted tech support and after some checking with other devices, tech support said it was a hardware issue and i should exchange my phone. Except i can't - i bought it used.

i thought maybe it was the vpn, so i tested a few different VPNs and none of them woudl stay connected for more than a few minutes except Onion VPN with Tor.

Does anyone ahve any idea what this problem actually is? can it be fixed?