Form not returning "Sent" or Error". But does send the email

Can you replicate the problem using the default theme with all other plugins deactivated?

to be honest I am no expert at this and would be worried about deactivating things in case I could not get things working again. I just don’t understand why it would work Ok on my desktop but not on my phone – do you have any clues.

There is no magic bullet in these situations. This is just basic troubleshooting – a process of elimination deliberately designed to locate the root cause as quickly as possible by first removing the most obvious and common culprits via a series of (often) temporary steps.

Your theme & plugin settings are saved in your database and should be retained for future use.