It is because the AZTech router (same for other brand name) has a PPP Always on with configurable timeout option.
When this option is set, say to 60 mins, the router will disconnect when it has no activity for over 60 mins.
Please make sure the router option (PPPoE timeout) is disabled, or set to always on, or set the timeout to a very large number (please check your router manual for this setting value).