Sounds like you have some type of significant interference between you and the nearest tower. Getting a solid 5G connection is a rarity for most people (despite the lies that carriers advertise). The towers for 5G need to be very close together and therefore unless you are in a stadium or city with modern infrastructure, it's hard to find and keep a solid 5G connection. If you are also dropping to no connection at all, then make sure that you test the situation in an area that you know for sure has a direct view of a cell tower for your carrier. It's possible that at your home, there is a water tower, the construction of your building, or some other large object between you and the cell tower that is causing the issue.
The last thing to check is that your iPhone case is not the issue. It's easy enough to test by taking the phone out of the case and testing for a consistent connection.