Setup: I bought a new Arris Surfboard SBG8300 Modem/Router. My Forerunner 745 would not connect to the 2.4Ghz network no matter what I tried or how I tried to connect it. My router displayed the MAC of the watch as connected to the network, but the watch returned a No Internet Connection error. However, the watch would connect and successfully sync to other networks. This indicated it was a network issue and not a watch issue.
Testing: I set up two identical 2.4Ghz networks and incrementally tested every possible setting: Static IP, Channels, Bandwidth, etc.
Conclusion. My new router had WMM (Wifi Multimedia) turned on by default. The 745 will not connect to the internet if the network has WMM turned on. Tested this with a factory reset of my router and toggled WMM on and off. WMM was the issue.
Hopefully this helps someone. I spent a lot of hours trying to figure this out b/c there's not a lot of documentation for this specific issue.