You are getting this error because your ZD seems running on 9.6 version which only support TLS 1.0.
Modern browsers do not support TLS 1.0 and that is why you are unable to access the ZD.
Please try installing an old browser version which supports TLS 1.0 and then access the ZD from it. Once you are able to access it, upgrade the ZD to 9.7 or later version to fix this issue (TLS 1.0 issue).
Use Chrome Browser before version 48, FireFox Browser before version 44 as it support TLS 1.0 Chipher suite
This is also documented on one of our KBAs https://support.ruckuswireless.com/articles/000005847
Syamantak Omer | Community moderator | Staff TSE | CWNA | CCNA | RASZA | RICXI | Follow me at https://linkedin.com/in/syamantak-omer-5949b6124