LE uses multiple validation points and those IPs are subject to change without notice and are unpublished; So there is no IP list to add to a whitelist.
I think, based on the screenshot only, that you should be able to code the FW rule in a way that would allow anyone access to the challenge folder.
If you are unsure on how to best configure the rules, I would either do the research on their site or try a search engine to get a good explanation of those choices.