The screenshot you have provided is not very informative, thus, I would like you to attach a troubleshoot file to your question. Please, replicate the issue, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there. The logs in the file might provide more insight into why the interface is down. At the moment I would suggest to double check the pre-shared key used to authenticate to unify wireless network.
I would also like to understand your end goal better. By WiFi relay, do you mean that Teltonika device is configured as a client to a unify access point and all client devices, which connect to RUT955 are within the LAN of RUT device?
Or RUT955 acts as wireless extender to unify, where DHCP to the LAN devices is provided by Unify as in this example?