A couple of times whilst flying over the sea I have lost video feed to the viewer. I was several miles from the nearest towns and probably more than a mile from the nearest house. There were no obvious phone masts nearby nor were there any boats or ships about.

I have had a look through the Go app menus and the HD menu in particular and see that it is possible to turn down the feed rate to the controller to increase the reception range. Menu path = MC settings -> HD -> Custom  -> (make changes by selecting or sliding as approrpiate)  -> X.
However when I turn the feedrate down the change does not seem to "take", this is based on reopening MC settings etc. and looking at what is set in "Custom".  The changes etc. were all made in the house and I have not flown to test whether or not the changes have taken.

Am I mistaken thinking that "Custom" should then be showing the settings I chose or is there a mistake in the procedure I am following?

Was your aircraft also powered up when you made the change?  Many settings modifications require the new value to be stored in the aircraft as well as the controller to "take" and be modified.   Remember, you are trying to modify a parameter in the aircraft's transmitter.

Good point! Thanks!!!! I dont remember but probably not, I will try it powered up later.

I fully expected to be now typing "Duhh call me Homer" but I have just tried a flight and, as I understand the procedure, changed these settings whilst the drone was hovering about 120ft away, the changes don't seem to have 'taken'.
As yet I have not tried this at distance or over water so I can not, at the moment, say whether the changes take effect 'silently' or whether the "Custom" setting reverts to displaying the default settings each time it is opened .