Those logs are expected if you open the webgui to random connection attempts. It's not an indication of any sort of compromise.
You can test it yourself, just try to access some page before you login and you will see those logs:
Apr 5 22:02:16 nginx 2023/04/05 22:02:16 [error] 47504#100318: *72304 open() "/usr/local/www/somenonexistentpage.htm" failed (2: No such file or directory), client: 172.21.16.8, server: , request: "GET /somenonexistentpage.htm HTTP/2.0", host: "4100.stevew.lan"