I second this. I have had 3 sync modules for 1 year now, and none has gone offline on its own. Maybe one did, once, but I’m not even convinced about that. More likely it was the camera batteries causing the particular issue, because next day I had to change them out anyway.
Moreover, I have one where last winter, for complicated solar power reasons, November to March, I had to remotely turn off the power to both my sync module and router, every single day, for around 12 hours. Each day, when the power returned, by timer, the system came back up, exactly as it should, without any complex sequencing, or pressing reset, etc. Good job too, because at this site I would not be guaranteed to revisit it until March, many months later. If it had stayed offline, and not sorted itself out each day, the product would have been useless to me. Whilst one trip every now and again would not have been out of the question, every day would have been hopeless.
I only mention that too, because others have mentioned that on power cycling a sync module, theirs will not come back up and obviously mine did multiple times, about 120 times. Since March it has never been turned off.
If you do get a new one, and it suffers the same issue, something else must be going on. At that point, as a workaround, you could get a smart switch to reboot your sync module. You said your router was unaffected, so you will be in contact with the smart switch, so you will always be able to reboot the sync module remotely. Better than waiting a month between visits, or waiting at the convenience of a friend, at least until you resolve the issue.