How do I restore a deleted shifter app widget on iPhone?

I had my shifter app as a widget on my Today View (the very left screen where you can place widgets). This app is like a calendar that displays my upcoming shifts for work. I don’t know if I accidentally deleted it or what but it’s gone and I can’t figure out how to get it back. I think it maybe got rid of it when I updated my phone but if anyone knows how I can get it back or maybe another shift work app that is compatible with the newest update as a widget please let me know.

[Re-Titled by Moderator]