Any time I make a change that effects the WAN side, it takes 6 minutes for the router to come to ready and even then I have to manually select the DISCONNECTED link to get the WAN port to connect and get a WAN IP address. Ya it came loaded with most current FW.
Your ISP might be sensitive to DHCP requests. Change DHCP mode from Aggressive to normal on the WAN page.
I would also avoid using Traditional QoS, as it's currently broken on the RT-AC5300.