How to fix iPhone SE only taking low resolution photographs
The iPhone SE camera cannot take pictures at a 1150×1530 pixel resolution – so something is not adding up here. By default the back camera should produce images at 4032×3024 pixels. 

I would suggest the following to ensure that you're grabbing a correct picture:

1) Use the built-in camera app

2) Ensure that no other apps are running at the same time (especially nothing should be used the camera for video)

3) Ensure that the dial at the bottom of the camera view is set to "Photo"

4) Ensure that you're using the back camera of the iPhone (i.e. not "selfie mode")

Take a photo and check its resolution using this method:

1) Open the Photos app and find your photo

2) Press the Share button in the lower left corner

3) Choose "Saves to Files" and save it on iCloud Drive or on the phone

4) Open the Files app

5) Find the photo in the list

6) Long-press the photo to get a popup menu

7) Choose "Info" in the popup menu

You can now see the actual resolution of the photo listed.

I have a suspicion that you might be sending the photo to someone else or a computer or similar using MMS, Messenger or similar, and that program is altering the resolution before you check it.

Therefore it is important that you transfer the photo using a method that preserves its full resolution.

That could be for example using iCloud Photo Library to sync it with your computer, or it could be using AirDrop to transfer the photo.