I don’t think that you need a special configuration on the reverse proxy.
It should be enough to reverse proxy to the NS8 IP like http(s)://192.168.0.1.
Anyway, for tweaking Nextcloud including persistence you could use occ:
runagent -m nextcloud1 occ
Example to output the trusted_proxies:
runagent -m nextcloud1 occ config:system:get trusted_proxies
I got 2 entries so array 0 and 1 are already used so to add a trusted proxy to array 2:
runagent -m nextcloud1 occ config:system:set trusted_proxies 2 --value=192.168.0.1
Delete a trusted proxy from array 2:
runagent -m nextcloud1 occ config:system:delete trusted_proxies 2
See also Using the occ command — Nextcloud latest Administration Manual latest documentation and GitHub - NethServer/ns8-nextcloud: Nextcloud module for NS8