What modem are you using? If you cannot re-connect w/o restarting the modem, than it would be the modem then. I never saw anything like that with ZS and I used it with a number of different modems over the last 6 years.
You are checking the status by looking at the update info page, which is not very reliable. Can you check ppp0 interface status and ping the provider’s router port instead when the problem occurs?