I'm having this problem with several different phones (BLU, moto G4) on Cricket. Reading through this thread, I find a couple of interesting things:
1) all of the IP addresses given in the reddit reference start with 192.168. Those IPs are private, and won't go across the internet. A router should simply filter them out and no response would be received. Why does this work for some people? Maybe the phone is automatically switching to LTE (or 3g or whatever over the air internet it can get) when it gets no response on wifi?
2) both "proxy.mobile.att.net", the alternate apn proxy in one of the replies, and "proxy.aiowireless.net" resolve to the same IP address (currently 172.26.39.1). This address falls into the range of 172.16.0.0 - 172.31.255.255 which is also private. But in August 2012, ARIN began allocating "172" address space to internet service, wireless and content providers. Given that the address range is private, I'd assume that most routers (including home routers) are configured to simply filter these addresses.
Thoughts?