It would appear that disconnects come from operator, considering the consistent periodicity and, in this case, not much can be done from the router's side.
One of the reasons for such behavior might be that very little or no data is transferred over these periods and disconnects are due to that.
As a countermeasure, you could try configuring Auto reboot instance without any action selected. This would ensure that some data is being constantly transmitted and help to sustain connection.
What you need to do is login to the routers WebUI, navigate to Services -> Auto reboot -> Ping/Wget reboot. Add a new or edit existing default instance. Enable it, and set Action if no echo is received to None, leave the rest as default, save and apply.
See if that helps.