Not OP , but I have seen type of behaviour , ISPs act like corporate firewalls and block everything by default just allow http on 80 and 443 in their consumer plan and try and sell enterprise/higher plans for literally anything else .

Not just that once a ISP was using a single CG-NATed IP for a ton of users, that the IP was constantly getting rate limited with captcha everywhere including Google Search and Cloudflare. They suggested I buy a plan with dedicated static IP instead of rotating a few more in their routers.