I know, but it's not up to me.
Sure it is. I think you missed my point completely.
I'll give it another shot. Let's say the server is located at 1.2.3.4 at the moment. You don't know what the finalized IP address will be, nor do you know what the actual domain name will be, as the customer haven't decided yet. That is not a problem. You could just register your own domain to 1.2.3.4, and use that domain in the app. So, for instance, you could register alex197project2020.com and point that at 1.2.3.4. In your app you just use that domain. After a while your customer decides that the actual IP for the server should be 9.8.7.6 and they will use the domain server.example.com. Their domain name is of no interest to you, you and your app will continue to use the one you registered. The only thing you will need to do is point alex197project2020.com to 9.8.7.6 and you are golden - no changes to the app needed.
A website's address should never change
I have no idea how you can say that. The IP address can easily change if you change hosting provider, for instance. (That's not an uncommon thing.) And that's not a big deal, as you just update the DNS record to point to the new provider.
your hosting provider should automatically update it's DNS servers
This makes me think you assume that you use the same provider for DNS and hosting. That is far from true for all cases. I would actually say it's more likely for smaller sites to have both those eggs in the same basket. I have lots of domains, and I use one provider to handle my DNS records, as they are doing a good job at
that. And I'm using another hosting provider for my many servers, as they are doing a good job at
that.