In Nextcloud, you will need to install nextcloud office app in the appstore, then go to the Parameters seek for Collabora Online tab under Administration
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You should set use your own server, and put http://localhost:9980, and check do not check ssl cert.
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Alternativelly if you have set a domain name, and the app was installed before, all should have been configured in nextcloud
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</tab>
</tab>
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then you will need to configure a domain name </tabs>