I haven’t actually touched any settings that’s DNS related or so, but since you mentioned it I’ll try and look for some tutorials on YouTube and download them so that when the issue comes back I’ll try the settings and see if it will solve it.
Your description makes me more convinced that DNS is related to that. You probably need to do more than just change the DNS settings on your network settings if your ISP uses a transparent DNS proxy. I used DNScrypt which allows me to connect to DNS-scrypt-supported servers and Acrylic DNS, not sure if those are the best options but it works for me. You can also continue using a VPN and find VPN-friendly casinos or just disable it when you want to access some casinos. I'm pretty sure most VPN apps support making exceptions nowadays if you want more QoL. CMIIW.
I’ll actually prefer to try your method of tampering with the DNS because the issue I face whenever I’m using VPN to solve it is that my internet connection speed becomes very slow.
Like if I should connect my VPN I won’t be able to do so many things because of the slow network speed but if I wait for everything to load it will work but it doesn’t really come up with the speed it usually does when I’m not connected to the VPN.
If I try this your method and I’m able to preserve the internet speed then I’d be so glad.