Ok lets try this, Turn OFF everything, accept for 1 wired LAN PC. Disable the 2.4Ghz radio. Leave the 5Ghz radio going. Save settings on the router. Power OFF the router and ISP modem. Power ON the ISP modem first then the DGL-5500 about 30 seconds later. Check the status and Internet function with your wired PC. If you have any other 5Ghz devices, graduate each one and connect them and test.
Then turn ON the 2.4Ghz radio, leaving the Samsung Galaxy Tab OFF. Test the 2.4Ghz radio with a few other devices.
Let us know if you see any other reboots during this process. Trying to see if we can help you narrow down where the problem could be coming from.
Any other WiFi routers in the area? Link> Use InSSIDer to find out. How many?
I have the SB 6120 and 50/2 ISP here. However I don't have any Samsung devices.
Keep us posted.