WNR2500 Wifi internet connection issues

We have a WNR2500 router for use in our small office. The router has one port connected to a switch for all the computers/printers in our office, of which there are 17. We also need to connect wirelessly via the router for the various tablets/phones people use. While it worked fine at first, lately we have been noticing more difficulty in connecting via the Wifi.

The signal strength is excellent (full bars), but we often cannot connect to the internet. Sometimes it connects, but only after a long delay. The other computers which are connected through the switch have no problems with connecting to the internet, only the Wifi connections. Testing the internet connection on the wired connections shows a connection speed of approximately 20 Mbps down/20 Mbps up. When the Wifi works it is approximately the same.

The Wifi signal never drops below full bars, so the connection to the router is fine, just the connection to the internet seems to be the issue.  I never see the tablet drop the Wifi connection, and the IP address appears to be stable.

I have noticed when I check the "connected devices" tab in the router that the device name and IP address fields are often listed as "unknown", and it changes every time I refresh. Not sure if that is a symptom or just normal function.

Ways I have already tried to address the issue:

1) I have tried power-cycling the router in the mornings, and that seems to help, but returns to poor function after 1-3 hours and has to be cycled again. Sometimes I have to cycle 2-3 times for the internet connection to improve.

2) I have tried changing the channel from Auto to 1, 6, or 11. No improvement. We are a stand-alone office with no one else nearby so I did not think this would help but figured Id give it a try.

3) Updated firmware to most recent version

What else can I do to address the issue other than continually power-cycling the router?  Or is this a possible symptom that the router itself has failed and needs to be replaced?