Swipe into your Control Center, turn on Airplane Mode for 10 seconds, and then turn it off to refresh your network connection.
Before diving into complex settings, try these three "reset" steps that resolve the vast majority of temporary sync glitches: why does my mail not download from server
Whether you're on an iPhone, using Outlook on a PC, or checking Gmail, here is a comprehensive guide to why your mail isn't downloading and how to fix it. Swipe into your Control Center, turn on Airplane