Same issue for me. My phone switch continuously from 4 bars 4G signal to 4 bars 3G signal and the other way around, with the phone still on the table. Every switch the internet connection drops.
Same sim, on an 6 years old sony stays always in 4G with no issues.
Same with my Samsung. Before s21+, I used s7, it kept a strong 4g signal everywhere. And now my s21 in the same places where I visit, can show 1-2 bars of cellular network, and instead of 4g the device switches to H+ or just H.