Is the issue specific to firmware versions starting with some certain version? Could you check, which firmware does not produce periodic disconnects?
Disconnects might be initiated by your service provider due to expired IP lease time and negotiation of a new address. In this case, there is not much that can be done.
They can also happen, if no traffic is forwarded through mobile connection over that period of time. You could try to configure periodic pings to some remote host to work around this. To do this, login to router's WebUI, navigate to Services -> Auto reboot -> Ping/Wget reboot. Create a new or edit existing default instance, enable it, set Action if no echo is received to None, set remote host address to ping, configure ping periodicity. See if that does anything.