Yea, buying a modem would be an option.
I wouldn't be sure what to buy though. I guess my options would be a little bit limited since I would need something that supports EuroDOCSIS.
It would probably be best if my ISP offered me something.
So I called up my ISP again.
Did the whole bridge mode <-> not bridge mode switcharoo again.
This time IPV6 behaved the same strange way regardless of the router.
Now they think, that they might have messed up their routing tables.
I wonder where this troubleshooting journey will take me.
At least my ISP is actually looking into this now.
It's always bad when your ISP blows you off because when you get to this stage in debugging it's often their fault and if they blow you off there is basically nothing to do 
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Just adding another experience to this topic, ISPs do mess up as well, especially with IPv6. Aside from the obvious braindead cases of not doing DHCPv6-PD at all (and only providing a single /64), they quite often don't test their updates, maintenance or reconfigurations/ restructuring enough. Case in point, I'm just now back to being fully operational, after the ISP broke all IPv6 traffic after an unscheduled maintenance window, despite still announcing IPv6 addresses and prefixes, after 3 days they fixed the /128 point-to-point address of the router, but left the announced routed /56 prefix broken (messed up routing tables, later looping routes, …), it took them 7 days and 14h (and lots of prodding from quite a few customers) to really fix the issues…
[Their own rented routers apparently have a hacky fallback to disable IPv6 silently.]
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Good news, IPV6 is working again!
I guess complaining actually worked for once.
Many devices that my ISP ships out to people come preloaded with a custom firmware.
The list of stuff that is broken on these devices is fairly extensive ...
Also I would like to add on a thank you for the help and reassuring words I received here 
The openwrt forum seems like a decent place to me.
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