Problems setting up a Reverse Proxy on IIS

Please forgive the newb question, but I’m a bit out of my depth with this one.

I’m running a Windows 2016 Server at home. I’m trying to set up nextcloud as a personal cloud. I have successfully installed nextcloud as a virtual machine on VirtualBox and can access it on my lan via (let’s say) 192.168.1.75. This takes me to the main web application. I can also access the Webmin interface through 192.168.1.75:10000.

I’m wanting to be able to access nextcloud through a domain name (let’s say me.org.uk). I have this

I followed this guide HERE to set up a reverse proxy. Now this works fine for 192.168.1.75:10000 – I can access Webmin via me.org.uk. However, if I try to set it up with 192.168.1.75 (to access the web app), I get a 500 – Internal Server Error. I also tried it with 192.168.1.75:80 – I get the same. Finally, I tried it with 192.168.1.75:443 (there’s a valid SSL cert against the domain) and get a different error:

# Bad Request
Your browser sent a request that this server could not understand.
Reason: You're speaking plain HTTP to an SSL-enabled server port.
Instead use the HTTPS scheme to access this URL, please.

I’ve also tried messing around with the site bindings, setting it to 192.168.1.75 for both 80 and 443 requests. Didn’t help. I’m not sure what I’m meant to set the bindings to if using URL Rewrite/Reverse proxy.

I hope someone can help as it feels like the last hurdle!