R6400v2 WAN port is capped to 10M/FULL.

I got a new internet connection with a static IP which is supposed to give me a 50Mbps/50Mbps down/up. I got this speed initially but the WAN port started capping itself to 10M/FULL. This issue would be solved if I restarted my router and it would establish a 100M/FULL link again. Later it got worse where the 100M/FULL link would only stay for 4-25 min and go back to the 10M/FULL link. 
I tried to use the router in bridge mode and direcly connect their modem to my PC. I briged the LAN ports 3 and 4. Connected the ethernet cable from the modem to LAN port 4 and the ethernet cable from my PC to the LAN port 3. Now LAN port 4 is capped to 10M/FULL no matter what I do. 
Please help.