Is there a reason why you want prowlarr and sab behind a VPN in the first place? If you just turn on TLS in sabnzbd you're not going to run into any issues and it's probably a better idea to search with prowlarr through your home address depending on what trackers you're using. I don't have either of these behind a VPN for my setup personally.
Gluetun just needs to share a docker network with traefik and not use network_mode. Do you have a default network in this compose file? If not you should add the same network name to every container and test to see if you can reach prowlarr with docker exec traefik wget -O - http://gluetun:9696.
Gluetun with protonvpn disconnects constantly for me and requires dependent containers to restart when it reconnects (https://github.com/qdm12/gluetun/issues/641) so make sure if you're testing stuff they're all restarted together
ISPs in Turkey will tell you a lie along the lines of "You will get 20% lower speeds on 3rd party routers" because they don't want to bother debugging issues on routers they don't control. I know that's not true since I get all the speed I pay for, but to connect at all I have to clone the ISP provided router's mac address because 3rd party routers aren't supported.
It's not impossible that your ISP could be doing something similar where they say they support 3rd party routers but throttle speeds based on your router's mac. If you do end up asking for an ISP router and get better speeds on it, you could try cloning the mac and see if that solves your issue for your own router.