No modem firmware and stuck on searching

Hello, my iPhone 5s shows searching after booting up and after 30 seconds it says no service or sometimes Cellular update failed error. I checked the about page in Settings-General and it does not show modem firmware but it shows IMEI. When I go to the phone app and dial *#06# just nothing happens. I remembered that my phone dropped in water like 1 month ago and nothing happened, but after 3 weeks I saw this error. When no service error started to appear, my phone sometimes was able to make or receive calls.

Any suggestions should be appreciated!

EDIT: I am using this iPhone as an iPod touch now and recently I noticed that every time I boot the device up it shows that Cellular Update Failed error even tho I do not have SIM card installed.