With the exception of the initial download, Glasswire uses barely any data at all. It will tell you what is using data, but only the computer it's installed on.
From what you've described you are on one of the legacy plans, with either a 7000 or 9000 modem and a daily data limit.
Smartphones, especially iPhones, are often the culprit of excess data usage. Your router is password protected, right? Have you checked your updates in your laptop to make sure that there isn't an update that is having a problem installing, therefore downloading over and over and over again? If you have Windows 10, there were recently some updates that were rereleased, and if your system can download it all, it might try again the next day, going into a perpetual data drain every day. It won't hold what it downloaded the day before, so it starts again the next day, having to download it over again. If the download totals 500MB, and you only have a 400MB daily limit...you get what I'm saying. Also, as mentioned above, satellite TV receivers can use a considerable amount of data, even when turned off, as they really aren't off, but in standby.
You can see what is connecting to your router by going into it's interface. The cloud also uses considerable data. Your Smartphones and/or your laptop could be connecting to their perspective clouds. There are a LOT of possibilities when it comes to unexpected data usage.
You may be able to drill it down by process of elimination. Start with one thing, preferably your laptop, then add the other things, one by one and over time, back to the network.
You could also try, if you are able, to go online during your download zone (2am to 7am). If it does happen to be a download that is too large to finish with your regular daily amount, it would be able to during your download zone, free data period.
Also, if during the two hours of unhooked LAN it showed no data usage, it's most certainly one of your devices using it, but that goes without saying. The modem isolation test normally asks for it the LAN cable to be unplugged overnight, but if it's happening whenever you go online, the two hour period would probably be enough. With that said, if you are able to not have your internet available for a night, here are the directions to perform the overnight modem isolation test, which is the same that you already did, but for a longer period....
This would give a definitive answer as to whether the problem lies with the modem, or your devices, but I can tell you with almost 100% certainty that one or more of your devices are using the data. There are also some other helpful things with regard to data loss in the thread given in that link. Again, it can be a pain in determining what it is that is using the data, but it can be done.
There are also programs out there for other devices that are similar to Glasswire.
Hope this helps.
Glasswire itself uses nearly no data, so I don't understand what you mean when you say "it eats a lot of data." For the last 30 days my Glasswire installation has used a total of 190KB. That's Kilobytes. Less than two tenths of a megabyte in 30 days.
Remember, you are in the download zone from 2am to 7am, so if your system starts to immediately use data at 7am, or whenever you get on it after 7am, you've got to figure out what is using that data. Glasswire is the tool for your computer, and it doesn't "eat a lot of data." You need to search for tools for your smart phones. It's also possible that your printer is connecting to an update or cloud service. Something is using the data. Hopefully someone else can chime in that has experience with Smart Phones and wireless printers.
The only explanation there is is that something is using the data. Why? I don't know. What? I don't know. Again, Glasswire for the computer. Other tools for your Smartphones. Unplug your printer for now to see if that's it. If it's unplugged and it still happens, you know it's not the printer.
If you would like help with your issue, you should start your own post. You can do so by clicking on the "New Post" button at the top of the page. List your issue, and also post either a case number from a prior phone call or the serial number from your modem. DO NOT POST your SAN (account number). If still available, the reps can pull your sales call to review it.