Help & troubleshooting for network issues, including connecting your device to your home Wi-Fi network, connecting to public networks, troubleshooting wireless issues & ethernet connections, and optimizing streaming performance.
Also, there have been a number of reports lately of people who have routers supplied by their Internet Provider having their router's 2.4 GHz band disabled by an update to the router software. They've had to have their Internet Provider re-enable the 2.4 GHz band, or in some cases swap in a new router.
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T
Assuming that they were working before, that your problem is that they won't logon to the network now and that you have access to your modem/router settings try this to test the situation:
Go to the wireless settings in your network (2.4 gig) and set the network security to unencrypted mode then see if you can log on or not. Do this just for a quick test and don't leave it that way except for the test and change it back as soon as you are finished.
If that works, look at your encryption settings and set them to AES (probably OK to use AES + TKIP as well but try AES first) and see that your passphrase for it is the same as the password you use to log on to your network, if not write the one in use down and then change it to your network password. If that works your problem is solved, if not then you are probably going to have to try a lot of things to find the problem.
FWIW, if your modem/router has a back up function for its settings then you should back it up before changing anything so you can always return to where you started if something goes awry and you don't know what change did it.