Dear Calleu
Someone from the community will definitely explain it more technically, meantime I'm sharing my experience with the same problem you have faced.
1. DSpace7 works on localhost without proper certification and FQDN but not on a production environment;
2. In a production environment (even in a LAN), the login and other administrative functions are not working without SSL and proper certificate;
3. Self signed certificate won't work for LAN/Production environment;
3. You need to initiate certification for your DSpace7 server (letsencrypt is what we have used);
4. Certification requires FQDN (IP is not enough), so you need to have a domain name first, require to map your IP with the given domain, and then you can utilize the letsencrypt process to generate certificate;
5. Your apache SSL configuration needs to point towards certificate that you have after initiation of letsencrypt;
6. Need to tinker proxy reversing process in Apache to redirect /server and / and similarly local.cfg in DSpace 7 particularly dspace.server.url and dspace.ui.url ;
7. Your environment.prod.ts in the frontend must be set on SSL mode - the rest section I mean;
8. Now Yarn or PM2 start will allow the frontend to talk to the backend;
9. Initially Yarn is a better option in case of error detection (but one terminal is engaged); and
10. PM2 gives a cool start without the issue of terminal engagement once it starts but doesn't give any hints of errors, if any.
Best wishes...