Diy solution :
Hardware : flight controller + companion computer (eg raspberry pi) + 4g dongle
Software : knowledge in linux => ssh to connect to companion computer, dyndns to bypass dynamic ip provided by your internet provider, udp redirect to redirect traffic, sakis3g or uqmi for dongle connexion, vpn to secure connexion
Complete solution : commercial link available in following links Connect pixhawk to raspberry and control it with 4G LTE
UAVcast-Pro 4G LTE Telemetry and Video