Hello @MikeKay,
welcome to the Nextcloud community! 
you setup looks good on the first glance. please dig deeper and review/share the logs from the client, reverse proxy, webserver and application to see where the redirect comes from. in the logs look at *forwarded_for* headers - these should tell the application how you access the system, you are missing trusted_proxies record so likely the application ignores
Please look at 101: reverse proxy and topics tagged reverseproxy similar discussions exist.