Hi,
I previously had DD-WRT running on my Netgear Nighthawk X6 (R8000) router in Canada for the last 3 years. I decided to move to OpenWrt today.
This router is connected to my internet provider's modem. I also have (another) WIFI Access Point that is connected to one of the Nighthawk's LAN ports.
After flashing the firmware, I could connect to the other Access Point through WIFI and access the internet, and so I know OpenWrt is correctly configured to at least give me wired access to the internet. I also logged in with SSH to the Nighthawk and pinged 8.8.8.8 with success.
I then tried to configure WIFI. The Nighthawk has 3 radios, one for the 2.4 GHz band, and two for the 5 Ghz band. After removing the default settings for the 3 radios, I created each of the 3 WIFI accesses according to https://openwrt.org/docs/guide-quick-start/basic_wifi.
At first, none of the wireless networks would work. Whenever I tried to connect my phone to WIFI, I got a "No internet connection" message. After trying some changes (can't remember which) and rebooting a couple of times, I finally got my 2.4 access working. But, the other 5 GHz WIFIs still give me "No internet connection".
Can anyone help ?
The current settings in the wireless overview are 
radio0: Channel 149 (5.745 GHz)
radio2: Channel 48 (5.240 GHz)
Both are set up for Canada.