What is the best way to install iOS app for employees in a company?

Disclaimer, I'm no iOS developer.

In 2019 my boss asked if I could make a small iOS app. 3 forms which will have CRUD operations. I did it because I wanted to learn iOS development. The company got an Apple Developer Account and it finished the project within the week. The app can be used only from work's wifi, so if you try to use it outside that network will not work.

The issue is license. When I install the app from my XCode to one of the employees' iPhone, the app will work for 1 year (if I have paid for Developer Account). Also every time I make an update, the employee have to come to me with the iPhone and I have to install it directly.

Yesterday I submitted the app for Testing using TestFlight. The app updates automatically, but today I got notification that the "Submission for testing is rejected", because the tester could not connect with the server.

My question is, How do you install iOS apps for your work employees? I do not want to get Enterprise account because that will cost.