The Internet in a certain application disappears after some time, what could be the reason?

Question

WiFi

Good day to all!

There is a house in which wired Internet has not yet been installed. A 4G router is installed on the roof, which distributes the Internet to home routers (MI-R3G, stitched on Padavan) via a network cable. Previously, a SIM card of one operator was installed on it, which provided favorable prices for internet packages. In this mode, the router successfully worked for a year without any problems.

But recently I found out that there is an operator that gives unlimited Internet in 4G networks, but with one caveat: you cannot distribute the Internet to other devices. For the sake of experiment, I decided to buy a SIM card from this operator and experiment, as well as find out if it is possible to bypass this limitation. After a little searching on the Internet, I successfully bypassed this limitation: The limitation was managed by editing iptables on a 4g router: iptables -t mangle -A POSTROUTING -o rmnet0 -j TTL --ttl-set 64

The Internet on the second operator worked perfectly, the speed was much higher than that of the first: in the first 3 days more than 200GB were downloaded. But then I began to notice that some applications on the phone began to lose the Internet connection after some time (25-30 minutes). And if you minimize this application and go to another, then it worked fine. The Internet did not work on this application. If you restart this application, then everything starts working. No problems were noticed from the computer, all applications work fine there. I myself do not understand networks at all, but I assume that the operator has some kind of limit on the number of connections.

Thank you for reading to the end, I would be glad to receive an explanation of this phenomenon, as well as possible solutions.