Did you do repair of avast via add/remove? Prior to having Avast what antivirus did you have? Did you use your previous antivirus removal tool before installing avast?
Simply move to broadband. No disrespect, but dial-up is now very old and long gone in this generation.
Are you running on an account with restricted privileges ?
Have you set the Update settings to Dial-up ?
AvastUI > Settings > Update - scroll down to Details and expand that section. Set it to ‘I only connect to the internet using a dial up modem.’
I’m hoping that may allow for changing the auto update setting and for it to stick - If the dial-up setting doesn’t stick either. Then an avast repair is the next step.
I don’t know why the settings don’t stick - this might possibly be permissions related, if you are using a limited user account in the Operating System (?). Limited user accounts would have limited functionality in avast, but I rather doubt that this should be the issue as that too would have been the same in avast 2014.
The auto updates and streaming updates shouldn’t be that large - typically streaming updates are very small KBs.
The virus definitions updates (VPS) also shouldn’t be that large probably hundreds of KBs up to perhaps a MB (depending on what they are covering). Normally there are only 2 VPS updates a day.
Downgrading to avast 2014 would do very little as that too has the same virus definitions updates and streaming updates.