xiakele Flarum reads the REMOTE_ADDR
value from the PHP SERVER_PARAMS
array.
I believe you can use Apache rules to rewrite headers in case the original client IP is stored in a non-standard header.
You can also write a Flarum middleware to read the IP from a different header or to use more advanced logic for the IP retrieval.
I'm not familiar with the service you mention but most proxy services will have a documentation that explains how to restore the original client IP for the most common webservers.