Software update 12.4.3 not working

I have an iPhone 5S. I'm being prompted to install iOS 12.4.3, but whenever I select Install Now, an error message comes up saying: "Unable to Verify Update - iOS 12.4.3 failed verification because you are no longer connected to the internet". Whereas in fact I am connected to the internet via Wi-fi. In the meantime, I can't send or receive texts and emails. I've searched for a solution online, but nothing seems to work. Any suggestions?