Hello,
Wireless mesh was introduced in firmware RUTX_R_00.07.01, thus you need to update the firmware version to at least that. However, I believe you have misunderstood, how mesh works. The idea of the wireless mesh is to replace cabled connectivity. It is a communication overlay for the wireless access points to communicate between each other, route traffic and transfer data. Wireless mesh has no impact on how end user equipment connects and communicates with the access point. Since you provide internet to RUTX10 with cable, you just need to configure RUTX10 and RUTX14 with identical SSID's and passwords.
Best regards,
Žygimantas