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Update: for whatever reason the annoying adverts are now much less frequent. Not sure if this is down to anything I've tried (malwarebytes scan?)
With regards the suggestion in buenchicos link, I need some tech help. To delete the ads it says "Go to the photos app, and delete all folders named unityadscache. Also to "manually go to your file explorer and search unityadscache".
Firstly, I've gone into apps and clicked on Google photos but can't see where I find folders?
Secondly, how do I go into "File Explorer" to be able to search for unityadscache?
Regarding alavahalf suggestion to do a factory reset, photos are set to auto back up to Google photos, SMS messages are also auto backed up and likewise WhatsApp as far as I know. How do I double check/manually do a back up and do I need to back anything else up, and if so how/where to. To be honest, I'm nervous about doing a reset.
I wish I could grasp this sort of thing quicker!!