A few possibilities spring to mind.
1. The antenna on your iPhone 13 is not as good as your iPhone 7, unlikely I guess.
2. Most likely - your new phone supports more frequencies than your previous phone and is now connecting to a different mast using a weaker band. You may be able to force it to connect to say 3G to see if signal is better by looking in the settings.
3. The signal strength is actually the same but the bars displayed on each phone are calibrated differently between different phones. Is the performance actually different between the two phones ?
