Have to be a little more specific as different Roku devices require different network configurations.
What Roku devices are we talking about (Roku Model # would help and detail...ie. can your Roku see the network and not connecting, not see the network, did it used to connect before, upgraded router, etc) and what steps you have taken to resolve problem.
To determine your Roku Model...
Normal Network troubleshooting begins with a router reboot and then a Roku reboot (Settings...System...System Restart)
Also, here is Roku's basic troubleshooting for Network problems.
Post back and people will be glad to help if they can.
3700x roku express,
I started at the link you sent, and found no resolve.
I've had the device for ~2+ yrs, connecting with no problems in the past. no change to our network, or router or any. It turns on, we can see the wifi networks all around, we find both of ours, 'guest' and our regular. both have 4 of 5 bars. We can enter the password, but when requesting to connect, it appears to be working for 10+ seconds before displaying inability to connect. I've restarted through the system settings, I've powered down/up, I've pressed the reset button. I've powered down/up the router. I've tried on different monitors. No resolve.
Multiple other devices are able to connect without issue.
Thank you for your help. -K
Check your router settings for the 2.4 GHz radio. This is the only band the Express works on. See if there is something labeled Legacy support, and if so disable it. If your radio is set to support 802.11b/g/n, try changing it to only g/n and see if your other devices still connect. 802.11b is extremely old, and unless you have something equally old it should work on the g protocol. This also has a benefit of making your WiFi a little more secure. You might also ensure your wireless security is using WPA2. If it's on WPA/WPA2, try just WPA2. Again, more secure and your Rokus will work just fine with it.
Echoing @atc98092 suggestions. Seems like Roku's 9.4 update may have affected some of the older expresses like your 3700. A little router configuration may be necessary.
For troubleshooting, if you are sure nothing has changed, the Express 3700 simply dropped off, maybe just disable your 2.4gz password (Under Wireless Security or the like , locate your 2.4gz, not the 5gz, and Disable Security). This way you are creating an Open Network, no password needed to connect. This will help to rule out if it is a simple authentication issue you may be having with the Express.
If your Express connects, then it tells you it is not liking your password (maybe some characters in it not being recognized and will have to come up with new password.)
Just make sure you Re-enable your Wireless Security after testing.
Post back and see if any of the responses so far help you out.