Similar experience here. I rely on mobile broadband for home internet and was okay for many years on LTE. Speeds up to 110mbps, at worst 30mbps during peak hours.
Around a year ago my carrier installed 5G equipment on a local tower (300 meters away), I got a €300 5G modem/router and for a brief moment got whopping 130mbps. Not the best purchase but at least I’m future-proof, I thought.
Then network stability went to the toilet. It’s a bit better when I disable 5G (🙃). I thought it might be my new modem but old one has same issues. My guess 5G equipment is not only overhyped but also buggy as hell. Obviously anecdotal evidence and ymmv.