HTTP_HOST with Reverse Proxy

I’m using Nextcloud on a debian VM on a Windows Server 2016 Hyper-V Host, whose IIS webserver also acts as a reverse proxy for a few local webpages. The reverse proxy is set up to redirect to the only internally reachable cloud.mydomain.local, which is my nextcloud-vm. Using the webinterface of nextcloud works fine, but when I want to set up the sync client on my notebook it starts to use the internal hostname cloud.mydomain.local, even if I entered the external one in the first place. After the setup I obviously can’t use nextcloud outside the local network.

Can I somehow suppress HTTP_HOST usage or force it to use the external domain? Overwriting HTTP_HOST seems not to work in IIS.

Thanks in advance!