Just connected to a different internet provider

That has nothing to do with your ISP or Internet Service Provider, it has to do with your cellular provider. Outside your house you should be using cellular. So if you are seeing no signal or Searching, you need to see this, If you see No Service or Searching on your iPhone or iPad - Apple Support or call your cellular provider.

WiFi only works for short distances. Generally about 300 feet for line of site, shorter if there are walls or obstructions in the way. Once you leave your house, your WiFi should drop off and your cellular will take over.

Obviously your wifi won’t work when you are out of your house but when you back in to the house your iPhone will be connected to the wifi

What do you mean WiFi doesn't work when you leave the house?

Changing your home ISP has nothing to do with connecting to other WiFi networks.

Follow the steps in the support link I provided, including contacting your cellular provider if you do not get a signal.