Network request failed

Hello 2 all.
I'm using Joplin Server 2.10.5, installed in docker on the Raspbian. So, the macOS client 2.9.17 works perfectly and syncs without any problems. But the iOS client 12.9.2 cannot sync with the server and gets this subject error.
Please, help. Is this problem solvable? Thnx.

This problem was solved by issuing a sert for my web-server. Before I used self-signed.