The DMZ for the ISPs router forward to the second router, then everything that hits your outside IP will be forwarded to router 2. Then on Router 2 you open the ports for your service and forward to the internal machine. That should all work fine.
Its quite complicated to setup as well, just went through the instructions and its a long way from just add to docker and run unfortunately. Would be nice to be able to just get a runner in the same or different docker and it just works easily without a lot of manual setup in Linux of directories and users and pipes etc.
I did the same move from contabo to Netcup. Contabo I had all sorts of weird bandwidth limiting problems that I couldn’t explain and which the continued to deny they were throttling. Netcup worked perfectly.
The UK is no where near the point of having too much power through the daytime. Today was pretty sunny, better than average day especially for time of year. At mid day there was still 5.8GW of fossil fuel use and 3GW of biomass, so about 8.8 GW of CO2 production. Or to put it another way of the 32.5 GW of power needed solar contribute 3.41GW.
There will come a moment where there is an issue where more storage is required to use that power through the evening and night or negative power pricing but its not the issue yet there still isn’t enough renewables to make it through a day without burning gas even on a windy sunny day so promoting more Solar and Wind is still necessary to get to netzero for grid power in 2030.