No Network when I'm outdoors

Do you have an active provider plan? If you do it’s likely the needed cell network is no accessible. Review the service map of the different cell service providers. You may need to setup a cell repeater which offers a mini cell tower as such which connects to your wired internet service. Placing it at a high point that serves your yard space.

The reason your phone is working inside is it’s leveraging your WiFI network.

As an example I live in a brick building which blocks the cell service from most of my home only when I’m near a few windows can I even get a signal which is not very stable. I setup a WiFI repeater setup as I don’t need the cell service as I have good coverage outside. Basically the reverse of your problem!