MTU (Maximum Transmission Unit)
Updated
The MTU is the largest packet a link can carry without splitting it, 1,500 bytes on most Ethernet networks. Tunnels such as GTP in the mobile network or IPsec add headers and reduce the room left for the device’s own data; RFC 4459 describes the problems this causes.
In practice
If small requests work but large transfers stall over a private APN or a VPN, suspect the MTU. Lowering the device MTU, for example to 1,400 bytes, or adjusting the TCP maximum segment size on the gateway usually fixes it.