I didn't see such a function on web interface of MR3420.
So if your ISP disconnects the data connection and you restart the router, the connection will be established again?

Actually the router connects automatically with my ISP, however this takes almost 60s. Due to the fact that the ISP disconnects every 25hrs (unfortunately not 24hrs) the time were there is no Service shifts during the day. If I had an automatic reboot function (or disconnect function), I could force the Connection to restart at say 3am and will I will have a continous Internet Connection during day time.

Unfortunately, no such a feature...

Hi Tony,

thank you very much for your Information.I will now have to install DD-WRT on the router. I was hoping to avoid this step, because the Firmware is stable (just lacking one simple Feature).

Update: Flashed OPENWRT on my TL-MR3420 which works perfectly. In Addition it has reboot capabilities.