Forward Nextcloud on the intranet

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Then it is clear, for your server 2 client not your user accessing it via LAN or internet, but rather is Server 1, that accessing it via IP address. This drives to situation when IP address being shown instead of FQDN. To do so, you needs to make Server 2 NC aware about proxy and Host name accessing via Server 1.
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Not sure, but it could be that you need to add this information via:

<Proxy "/">
  RequestHeader set Host server2
</Proxy>

    ProxyPreserveHost      On

    ProxyPass / http://192.168.1.221
    ProxyPassReverse / http://192.168.1.221

I believe there are few headers that you need to set. E.g. this is nice example and it is solved: Nextcloud behind Apache reverse proxy, remote.php becomes remote.phpp

Also check nextcloud config: