Tele2 wifi hub c2 subscription: 250/50

Yes

You don't. It's DHCP from the ISP.

What are you trying to achieve?

It's most likely possible to put the router into bridge mode. You can probably even do it yourself. The login to the router is on a sticker on the router itself. You don't need credentials to connect your own router. When you connect your own router to the bridged port you will get a direct connection.

But in my mind you only need to do this if you want to change settings in the router. I have myself most of the time just disabled WiFi in my ISP router and connected Unifi AP's for WiFi. The problem with most customer routers is that they are not getting updates for more than a few years. By sticking to the ISP router they will keep the router up to date and replace it if they see a need. Since I'm not using the WiFi of the router performance didn't really matter much. After doing this for many years I have however now replaced the ISP router with an ethernet only router that is semi professional.

Thankfully, ISPs in Sweden generally don't use PPPoE. Instead, it's just straight DHCPv4 for IPv4 and DHCPv6-PD for IPv6 (for ISPs that offer it).

You should be able to open ports on the ISPs router as it is, or on your own router connected to it through bridge mode.
As far as I know, Tele2 does not use CGNAT outside of mobile connections, they do in fact offer 4 public IPs if you have a 100mbit+ connection according to their site.