@Kevin_Z thanks for your response.

Below the answer to you questions:

1. How often does the disconnect issue happen? Does it happen on all wireless clients or some special devices? How did you recover the connection?

It's totally sporadic and only on the wireless clients. I do not do anything special to "recover" the connection: the connection is established just after a couple of seconds and without me doing anything.

2. Who is your ISP and how about the lights' status on the modem when the connection drops?

My provider is Forthnet (Greece) and the wifi light is always blinking (I do not see any difference in the lights when these short drops happen).

The same also for the "lan" connections: the light is always blinking. 

3. How many end-devices like our PC, laptop or mobile phone are connected to the modem, wired and wirelessly? Do they drop at the same time? 

In total there are about 15 devices registered and usually around 10 are connected at the same time. Out of these 10 devices, 3 devices are connected via wire, the rest are wireless. When these drops happen it impacts all the wireless clients and none of the wired.

4. Are there any heavy traffic applications when the network drops, such as downloading videos or playing games?

No, nothing special. Sometimes, even when I try to connect to the modem (192.168.1.1) I get these short drops (which is a connection without any significant traffic).

5. Have you tried to reset it to factory defaults and reconfigure it from scratch, then confirm?

Yes, I have done that as well and I have not observed any difference. What I noticed though is that when I reconfigure it from scratch, the PVC04 virtual circuit is used (by default). Since 2 days ago I reconfigured my internet connection to use the PVC0 instead (I think I read somewhere that it might be better to setup the connection to that vc instead of the 4). I did not observe any difference to be honest, but just mentioning it.

@Kevin_Z thank you for your time Kevin!