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.