I have DrayTek 2820VN and BE (Bethere) as a service provider.
During the last few months I get a lot of frequent disconnections and it lasts for couple of hours. It works fine for couple of weeks then again I get disconnections for couple of hours (about 3 hours) every day for 2-3 days
.
When I monitored the online status page on the router’s page, I have noticed that the router connects to the ISP and the connection lasts for about 30 seconds and then it disconnects again.

This seems to be the case with Draytek router only. When I connect BeBox or my Netgear routers to the same line I do not get any disconnections.

I have just upgraded the firmware on the router to 3.3.4 but still get the disconnections.
Using the following settings
MPoA/1483 Bridged LLC
ADSL2+(G.992.5)
UBR
VCI 101
VPI 0
Mode ARP detect.

Is there any reason to why do I get disconnections?

If it's only the draytek doing it then check whether the ADSL drops at the same time, if it doesn't then try changing the WAN connection detection to ping detect and have it ping an IP that you know will always work (8.8.4.4 is my favourite).
If it still drops but doesn't with the other two routers, that would indicate (I think) that the connection is actually dropping but the other two routers are less aware of it than the draytek.

If it is dropping the ADSL when the disconnections happen, try a different modem code firmware which you can get from the draytek.com website, 211801 is best if you've got a long / low speed ADSL line, otherwise try newer annex A modem codes (higher number = newer)