Why can’t I get more than 1 bar on my Iphone 13 pro?

Could be a number of reasons. The most logical initially would be the service in the particular area or building you are in isn’t strong. You can restart the phone to see if that helps. Check to see if there are any carrier updates. If the issue is in a particular building (house, work, etc) connect to Wi-Fi and activate Wi-Fi calling (if your carrier supports the feature). Otherwise contact your carrier to try and troubleshoot the issue

That’s most likely a cellular problem with your carrier