for future user who happen to come here having the same issue, I don't find any solution for this. But I found some trick

When I click backup, browser send request to server. in the browser, I can copy the request as curl. 
open google colab, type ! (exclamation mark) followed by the copied request

for example 

!curl 'https://www.awesome-website.com/web/database/backup' \
-H 'Connection: keep-alive' \
--data-raw master_pwd=your_odoo_pass\&name=your_db_name\&backup_format=zip

Then I get error if I run as it is. Curl need -o flag to determine what filename will be if the download finished. So here is the full command

!curl -o your_filename 'https://www.awesome-website.com/web/database/backup' \
-H 'Connection: keep-alive' \
--data-raw master_pwd=your_odoo_pass\&name=your_db_name\&backup_format=zip

Then run it

Google will download the database with high speed internet. Then I can download with my crappy wifi :D

after the download finish, I can either copy it into my google drive for me to download it later or download it right away

Please be careful when using this trick because this trick will expose your odoo password

Hi,
It seems the backup from database manager page is stopped/terminated by NGINX set up, may be max file size or any configuration is not allowing to take a backup more than 1 GB, either you can adjust the NGINX configuration to allow more size or take backup using pg_dump commands.

Thanks