To use the ZeroSSL web-based client, you should be able to adapt the instructions given in my post in another thread: Simple and Clear Directions
I think that using certbot in manual mode on you local machine is likely to be less hassle than the web-based version.
Also, check with your host and see if you can upload the needed files with FTP. If so, try using the GetSSL script to automate the process for renewals.