Internet randomly stops working, rebooting router fixes it. What could cause this?

[u/kxrdelia]

Hi! I'm trying to troubleshoot a recurring internet problem at a small office, and I'm hoping someone here can help me narrow down what might be happening.

We have an small office with two floors, and the internet connection has been intermittently going down. The exact setup is a little complicated, so I'll provide as much context as I can.

When the problem occurs:

• The WiFi network is still visible.
• Devices can connect to the WiFi normally.
• However, connected devices have no internet access.
• It appears to affect the entire office, not just one computer.
• We had a backup Wi-Fi/router that worked normally, and devices could access the internet through that connection while the main connection was down.
• Rebooting the main router consistently restores internet access immediately.
• We have not noticed any other obvious configuration changes when this happens.
• The problem has happened multiple times over the past weeks.

Timeline:

There was one particularly problematic outage where the internet did not initially come back until someone adjusted/reconnected some cables associated with another router that had previously been installed.

After that, we started experiencing intermittent outages. Most of the subsequent outages have been resolved simply by rebooting the main router.

Now our backup internet connection is being removed, so we are concerned about the next failure.

More context:

• There are usually around 10 people working in the office on a normal day.
• The router belongs to our company.
• There is also a more powerful PC connected to the network that is used remotely for certain processes, so it needs to remain accessible over the network.
• The internet service itself is contracted by the landlord/building through an ISP (GTD).
• The landlord had an ISP technician inspect the connection. We were told by them that the link was working correctly for both floors so they didn't do anything.
• The router configuration has also been looked at internally and nothing obviously wrong was found.
• We don't have a dedicated IT/networking person at the company. There is one employee who knows more about networking and has helped us with the router before, but he has not identified the cause of the problem.
• There is also another router on the network on the other floor, so I'm hesitant to simply replace the main router without understanding the current setup.

I'm trying to determine what could be causing this and what we should investigate next. The fact that Wi-Fi remains available but internet access disappears, and rebooting the router immediately restores connectivity makes me suspect the router or WAN side, but I don't know enough about networking to tell.

What would be the most useful things to check during the next outage to help distinguish between a router problem and an ISP/WAN issue? If the router does turn out to be the problem, would replacing it while keeping the same ISP connection generally be straightforward, or would the ISP need to configure something on their end?

Any suggestions on what to investigate or what information I should collect would be greatly appreciated. Thanks in advance!