Yuki dropped his iphone at “the” riverside.

First I have a favor to ask. I always confuse how to use the or a correctly.
Is the title a correct sentence?

Well, because of the coronaviruses, we can't meet my mother- in-law as of now.
So one day our schedule was open, and decided to go out to  --- check fishing points in a rural area.
Here in Tochigi, since March river has been open for armature fishermen. (Or we can say,  at most of rivers, usual people are permitted to fish for a  particular period buying an annual piscary. (Piscary, is it right?)

I'm not interested such fishing points, but near a fishing point there a spring and we can get some water freely. To tell the truth people can get three bottles of water.



On the way home, we dropped by a little dam.
Until there, he had his cell phone.




And we took a rest at a farmer's shop.  He noticed that he didn't have his cell phone.
From the dam to the farmer's shop, we dropped by three fishing points.
Anyway we went back to the dam. It wasn't there.
I had my cell phone, so I called to his cell phone. It was ringing, so it was still somewhere.
We went to the next point, it wasn't there.
And we went to the third point, it wasn't there.
We are really in our old generation, we were looking for it with very old way.
I noticed, our cell phones are IPHON.
So I tried to iPhone app. --- but we couldn't operate it correctly.
And I also noticed, our cell phones are normal cell phone, not budget smartphone.
So I called the telephone company. Then we understood we had already obtained insurance to search our cell phone. They looked for his cell phone. According to the GPS it must be in the second point.
We went back to the second point again, and looked for it again.
We found it!
Nowadays to save our expense, we are thinking to change our iPhone into "budget smartphone"
However for a while, it seems to be better to use there iPhones supported by a major company because we are in the senior generation.