Not all of it is your ISP's doing. Don't forget that your ISP connects to upstream providers and the problem could be on their network too. In that case, your ISP can only pass on the info and wait till the upstream fixes it.
There may also be other factors, like BGP hijacking. Recently, an ISP from India hijacked complete networks from another part of the world because of a typo. This got noticed and fixed in a matter of hours, but meanwhile, the people on the other networks couldn't reach a large part of the internet.
And it is absolutely not clear to me why one browser would work and another one not on the same computer. That could have to do with SSL settings, as browsers these days no longer have the same settings because of security. To connect to the forum, you need to authenticate, if you have auto logon set to "on". In case you have problems connecting to the forum, you should set auto logon to "off" in your forum preferences. But it can't play with RME's website, as there is no authentication going on there.
Then there is another thing: both Apple and Microsoft changed the order in which your DNS servers in the list are used. It used to be top to bottom of the list. Now it is "bottom to top". I have no idea why and I already forgot when it started. I think Apple did that starting from Lion and to cater for a number of new features in Bonjour Proxy. If you have several DNS servers listed, it could mean that the least reliable is used first. Most people will get the DNS from their router through DHCP, though, so this should only be valid for people who have manually configured DNS settings. There are also an important number of hijacked routers around. And these will first show because of connection problems. The goal there is to click on invisible (to the user) ads and make money that way. As routers are usually on 24/7 there's more money to be made than by hijacking computers or browsers.
And then there's Cloudflare. Their DDOS protection is on automatic. They'll block entire ranges if needed to protect a site that is being DDOSed. If RME's server resides in the same block and the attacker seems to reside in your block, the connection will not work. And since the attackers usually use a botnet, that could happen to any customer of any ISP. All ISP's have infected computers amongst their customers...
But the center of the problem is ipV6...
If you'd want to report this to your ISP, you need to document it thoroughly, include reports from MTR fi, dates and times when it happened, and do it by mail.
The many call agents that work for your ISP do not understand these matters and if they need to write it down to escalate, they'll make errors. Second or third line support will not understand and will not pass on the info, unless they get a ton of reports. And that won't happen, as it is a rare problem.