If your main goal is to extend WiFi coverage with the TAP100 connected via a wired connection to your RUTX50, the setup is straightforward, and you don’t need to use the Mesh configuration.
Simply connect the TAP100 to the RUTX50’s LAN port – this way, the DHCP server (RUTX50) will assign an IP address from its lease range, and the TAP100 will be in the same subnet. Then, define the same SSID and password for both APs, and this setup should be sufficient for your use case.
Channel for TAP doesn’t require to be the same as RUTX50 2.4 GHz band’s, instead, you can use a less crowded one in your environment to decrease wireless network interference. A channel analysis scan can be performed on RUTX50, navigated to Status → Wireless → Channel Analysis page.