You might want to read this thread:
I realise that it’s not exactly the same scenario, but as @Gunner has said, this looks like a mobile hotspot:
I’m guessing from what you’ve said that it’s actually a router that has a 3/4G SIM card in it. In which case, the conclusion from the linked thread that
Vodacom South Africa 3G data connection, is blocking Blynk-cloud (PSH flagged) TCP/IP login binary-payload packets when my NodeMCU attempts to connect via WIFI tethered smartphone
may also apply to your situation. Vodacom appears to be a subsidiary of Vodafone, so I’d think that is where your issue lies.
You should still use a baud rate of no more than 9600 with SoftwareSerial, and a separate PSU with common grounds for your devices if you want consistent results with the hardware you’re using.
Pete.