There are a number of different reasons why Internet connection issues can occur, so unfortunately there is rarely ever a "quick fix" for problems like this.
One of problems can be the age of the router. About how hold is the AirPort Extreme? We ask because the useful life......on average.....of a router designed for home use is about 5 years before they tend to start to develop problems. Loss of connection is typical in an aging router.
If you are unsure about the age of the AirPort Extreme, please tell us whether it is shaped like a "tower" or whether it is shaped like a small white pizza box.
If your AirPort Extreme is shaped like a small white pizza box, please look on the bottom of the device for the model number.....usually embedded in the foam pad base. The model number starts with an "A" followed by 4 numbers. Post back that info when you can.
We assume that you have already tried another Ethernet cable to connect your modem to the AirPort Extreme, correct? If not, a poor electrical contact can certainly cause a loss of connection because 16 tiny copper contacts must all line up exactly for a good connection to occur. Corrosion of the copper is the most common problem, so if you have not swapped out the Ethernet cable in some time, that would be a good thing to try.
Service providers tend to always insist that "everything looks good" from their vantage point, so you might want to check their claim by connecting your Mac or another computer directly to the modem for at least a few hours to make sure that the connection holds solid that way. A longer "test" period would be better, if possible. If the connection appears to be reliable, the wiring from your provider and the modem would appear to be OK.
Some online learning sites can use up a lot of bandwidth, so if your Internet connection speed is on the slow side, that might be another cause of the loss of the connection. When things get overloaded, the connection will drop. One of the online sites that my granddaughter is using specifies a minimum recommended Internet connection of at least 25 Mbps for example. Her parents had to "bump up" the speed of their Internet plan to meet those requirements. How does your Internet connection plan compare to this?
If the Internet connection checks out OK during your "test", you might want to try using a different DNS Servers like Google for your Internet connection rather than the servers that your ISP is using. Post if you are interested in trying that, but if there are other issues, changing the DNS servers will unfortunately not help the issue, so this is probably one of the last things to try after other measures have not been successful.