A few people with this problem have reported that they ended up using a different phone (from a family member or friend) to install the camera and for some reason that works.
Otherwise you best be is to contact Google Nest and speak to a technician that can help you. Here is the link to open the ticket and speak with someone. I suggest you allocate about an hour for this....
If they get you up and running please return here with a few details about how they got it working.
Hi, I am sorry that I haven't replied; I am very thankful for the advice provided, but I haven't been able to test it yeet, since I asked for a friend who lives in a neighboring country – she's been away traveling and as soon as I visit her later this week I will try out the suggested solution. Thanks for your time, you've been very helpful.
problem finally solved; after Mae at the Google Nest Customer Care Team suggested I'd do a factory reset if nothing else worked. Turns out – which I didn't know – that the doorbell had been up and running before, but refused to get configured after charging which was probably because my friend had switched to another internet provider in the meantime; and the device stayed "locked" to the old wifi network. Thanks so much for your help,