Thanks for the info, this really helps.
The A1354 was discontinued in the summer of 2011, so the product is at least 9 years old and probably older depending on when it was first placed into service, which might have been as early as 2009. On average, the useful life of this model was about 5-6 years, so you have had very good luck with the product.
It was the power supply that caused the most issues for users on this model, since it tended to slide downhill starting at about the 5-6 year point. As the power supply aged, it had a tendency to produce power supply "blips", which could often result in a loss of the Internet connection until the AirPort was restarted. Things were fine until the next "blip".
If you want to gamble $15-20 dollars on a replacement power supply, that might bring a few more years of use for the AirPort. I've used the adapter linked below on some older AirPorts with very good luck.
https://www.amazon.com/T-Power-Adapter-Compatible-Airport-Station/dp/B008H7MFYC/ref=sr_1_1?dchild=1&keywords=power+supply+for+a1143&qid=1591898127&sr=8-1
Of course, here we are assuming that the modem is not the cause of the issues. If a new power supply does not help, you can usually return the product for credit, so your risk is minimal.
If you want to check to see if the modem might be the culprit, you can connect your computer directly to the modem for a day or two and check the Internet connection from time to time. If the connection remains stable, the modem is fine. If you see that the Internet connection drops during the "test", then you may have a modem issue and AirPort might still be OK, even though it is probably overdue for replacement, or at least a replacement power supply.
If you don't want to fool with trying a new power supply or spending time on a modem "test", your best bet in terms of simplicity and reliability is to check with your ISP to see if they can supply you with a modem/router, often called a gateway.
This is a product that combines both a modem and router in one single package. The product is optimized for the service from your ISP, so you don't need to worry about potential compatibility issues like what router works well with which modem. There is also practically no set up required for the device, it's usually pretty much plug and play.
Online, it shows the plan provides 20.6 gigabytes
Not sure what this is referring to since Internet connection speeds are measured in Mbps. Try connecting to a site like www.speedtest.net and run a few checks to see what the results might be.
If you don't want to look at a modem/router from your provider, then you can go with a separate router to connect to the modem, assuming it checks out OK in your "tests". We don't want to suggest a potential replacement router though, until we know what connection speed plan that you have and whether or not you might be upgrading to a higher plan in the future.
Please post back when you have run a few checks using the Speedtest site linked above.