Still struggling with Monterey Update

I have followed all the troubleshooting instructions of a former thread about updating to Monterey, with no results.

I have erased my 2019 MacBook Air and reinstalled Mojave from Network Recovery.

I have tried to update from my home internet connection (ethernet to router), but have always received an error message of aborted download.

I have my connection checked, everything's OK.

I have tried with the internet connection at work, it's known to be reliable.

I have received the same message error.

Furthermore, the download speed is always painfully slow.

What am I to do?

Also, I cannot transfer data from my 2014 MacBook Air to the more recent one because the older one has Big Sur installed ( and I should update the 2019 MacBook...how?)

Is there any way to get the 12.6 GB Monterey update in another form?

Is there any possibility to get access to a faster Apple download server?

This is the first time I really don't know what to do (and I have a long experience as IT professional and with Macs, as I have explained in the other thread...)