ZYXEL GS1350-18HP SSL CERTIFICATE PROBLEM

Its not a problem its just a default certificate to which you can add your own when you login there are some ways to do this.

If by IP like 192.168.0.135 for the switch you can make a self-signed then you will need to add that certificate to your PC and switch

A better way is to get a DDNS like GS1350-18HP.ddns.net then make a at Free SSL Certificates and SSL Tools - ZeroSSL have your local DNS point GS1350-18HP.ddns.net to IP 192.168.0.135
you will then need make the certificate.crt and certificate.key to a .pfx to the switch
page 343
https://download.zyxel.com/GS1350-26HP/user_guide/GS1350-26HP_V4.90_Ed2.pdf

use windows to make .pfx

certutil –MergePFX certificate.crt cert.pfx

or open SSL Win32/Win64 OpenSSL Installer for Windows - Shining Light Productions

openssl pkcs12 -export -out certificate.pfx -inkey certificate.key -in certificate.crt