Check this pagehttps://www.w3schools.com/asp/coll_servervariables.asp
It's the server (IIS) who fills in this data for ASP. It does that based on the IP of the client that connected to it. This will be the reverse proxy. REMOTE_ADDR is not based on a received HTTP header, but is based on the TCP endpoint. This is determined by IIS.
You need to either
1. alter the ASP app to use a header that the reverse proxy passes to you instead of the server variable.
2. get IIS to give you different information in this server variable. It would have to get that from the header the proxy fills in. I don't think IIS can do this, since it gets the headers only after the request is made, but I could be wrong, I'm not expert on IIS.