Brother printer won't connect to KVD21 Gateway

The Roku TLC TV is one I have not had any dealings with but I suggest looking up the specifications for the wireless interface on that and make sure of the authentication type it supports. The gateway frequency must also have that authentication type, of course, to be able to connect.

The printer will probably have to be on the 2.4 GHz frequency alone. You will probably have to add another SSID for 2.4 GHz and join it to that. I really believe in most cases it is the authentication that is not matching but I have suspicions that the band steering of the 802.11ax implementation the router uses just does not play nice with older equipment. Well, the older equipment's drivers are just not designed to work well with the newer standard and vise-versa. 

If you want to not have to fight with the wireless just use the Ethernet port if you printer is close enough to the gateway. 

IEEE 802.11b/g/n (Infrastructure/Ad-hoc Mode)

IEEE 802.11g/n (Wi-Fi Direct™)

Wireless Network Security 

WEP 64/128 bit, WPA-PSK (TKIP/AES), WPA2-PSK (AES)

If you have the inclination check out the conversation, Can't access system settings  I posted information from the Nater Tater YouTube video that provides helpful info to see what is going on with the gateway and how to use the configuration extraction, edit and update to the gateway to make changes needed. Currently it is really only possible to do configuration with the Arcadyan via the mobile application as the web UI does not present the administrative login. This is seen with the newer version of the firmware. 

If you have the technical drive and want to give it a go the info is there. You can also go out and watch his video which is very helpful. That should allow you to get things working.