For Ilkka and anyone else interested, you can find a (long) thread on DPReview where a solution is proposed, then others comment (not necessary to follow the whole thread through!)
https://www.dpreview.com/forums/thread/4114865
Note: this thread focuses on the D500, so benefits or options may be different on other models
I don't have a camera standby timer timeout related problem. The wifi connection is formed reliably as long as there is no significant source of external radio interference (ie. being in the vicinity of 1000 mobile phones would be such interference and Snapbridge would probably not connect).
The sequence that works for me is this: I go to snapbridge, I select the correct camera and set it in wifi mode. Then I go to the camera and select establish wifi connections and go to settings/wifi in my iPhone and once D850 appears in the list, I click on it. I then wait a few seconds until the connection is formed. This typically takes more time than my standby timer on the camera is set to (10 s) but this doesn't cause a problem. However, the app must be set in Wifi mode before I start the sequence otherwise it won't work. Also if I click D850 in the Settings/Wifi and leave Settings to Snapbridge before the connection is formed, no luck: the connecting process is interrupted. I must stay in settings/wifi until the connection is established and only then go to Snapbridge. You could call it a secret handshake, I suppose, there are so many ways in which the user can do this wrong and it seems a lot of it is related to the mobile device not multitasking properly.
I mistakenly thought the dropping of the wifi connection is dependent on how long I am idle. This doesn't seem to be the case. As long as I don't leave the Snapbridge app screen, the connection is maintained. If the phone wants to access the internet in the meanwhile, it asks me if it is ok to connect using mobile broadband instead of the default wifi, and I say yes it is then it still retains the wifi connection to the camera. However, if I switch to another app then the wifi connection to camera is immediately lost. This seems to be my main problem: I want to go to the photos and send them and then I would like to continue transferring more images but the wifi
connection has to be re-established since I left the app screen.
So: for me it is not a timeout problem; I waited a long time with the Snapbridge screen open after transferring the previous file, and more files could be downloaded and remote photography activated as long as I did not for a second leave the Snapbridge screen.
When making the wifi connection from the D6 to a laptop or desktop, this was not an issue; the connection was not lost by using applications on the computer. This is how it should be in the mobile device as well. I want to multitask, I've been doing it for 30+ years and a connection should not be lost merely because I use other apps on my device.
Anyway, I am gaining a better understanding of what works and how it can be used successfully and what kind of user behavior leads to connection failure. This has been a good discussion.