iOS 9.3.5, iPhone 4s, email stopped

On Sat Oct 1st, my iPhone suddenly stopped receiving all emails that were sent to one of my gmail accounts (my main email account).

I could receive other emails (including one other gmail account), and, the main gmail account emails were still being received fine on my iPad (also running 9.3.5), and on my iMac. And I could send emails from this main gmail account from my iPhone.

As far as I know, I had not changed any settings or anything. I checked my email settings, and could find nothing that looked wrong. I compared my iPad settings to my iPhone settings for the problematic account; they were set to use a different port, but otherwise the same. I tried re-booting the phone several times. I tried searching online for this problem.

To make it even stranger, about two days later, I received on my phone a few emails that were sent to this gmail account, which were sent the day that the mail stopped coming in (in other words, I started receiving a few two-day-old emails). This pattern has continued for the last several days, where each day I will receive a few more two-day-old emails from that main gmail account. I never get one from the present day, even though they show up on my iPad on the day they are sent to me. So on Tues, I get Saturday or Sunday's mail, and on Weds, I get Monday's mail, etc. Meanwhile, my other devices are having no problem with this gmail account, and receive the emails promptly.

My iPhone is now no longer reliable for receiving email from my primary email account, which is a huge drawback. Anyone else having this issue, or heard of anyone that has had it? Any suggestions would be greatly appreciated. Thank you.