I need to access a third party API via server side that requires IP whitelisting but WIX can't share a list of IPs. Any suggestions?

Try these pages:

Wix Code: Accessing 3rd-Party Services with the Fetch API

Third party API

IP address of requests stemming from backend web module

I have asked someone to setup a proxy server using Squid.

Question: how do I make a fetch request on the wix web module via a Squid proxy?

Have a read of these pages.

Wix Code: Calling Server-Side Code from the Front-End with Web Modules
Wix Code Tutorial: Sending an Email on Form Submission
Wix Code: Accessing 3rd-Party Services with the Fetch API
Connect wix website to a server
Nginx proxy Wix site shows only blank page in browser or not found

Any luck with this?