I only get one bar.

Take the phone out and drive around and watch the cell signal strength. If it never changes, then you may have a SIM card issue. Contact your carrier. If you get a better signal during your "test", then you have 3 options:

Option 1.......Move the phone closer to the cell tower

Option 2......Move the cell tower closer to the phone

Option 3.......Move both the phone and the cell tower closer to each other

See these Apple articles below:

If you see No Service or Searching on your iPhone or iPad

If you see No Service or Searching on your iPhone or iPad - Apple Support

Manually update your carrier settings on your iPhone or iPad

Manually update your carrier settings on your iPhone or iPad - Apple Support

Update your network provider settings manually on your iPhone or iPad

Update your network provider settings manually on your iPhone or iPad – Apple Support (UK)

For cellular connection? Most likely you're in an area that's not covered so well by your cellular network, I have the same, in my home I always have one bar of cellular. Outside of my neighborhood it gets better. check that first.

If you always have 1 bar of cellular connection, contact your carrier. It can be that you need a new SIM card