I selected ‘add default route’ when using the SSTP client, but it is not working. Does the server need to push the route, or do I need to configure something else?
The routе is added to the main table, but it will not be active because you already have a similar path to have Internet - only one path with the same destination can be active. If you want the clients to use sstp, another route table is adding in which you manually add the static route via sstp connection and then you add a rule that defining which clients or networks will use/look the table!
You are right. I did indeed manually add the route. It seems I misunderstood the explanation in the text, as I thought the default route was 0.0.0.0/0. But the distance would be the same, so the routes might not work as I intended. Therefore, I split it into 128.0.0.0/1 and 0.0.0.0/1.