Here is their response to my support ticket enquiring regarding changing the nameservers:
"Dear Ladies and Gentlemen,
thank you very much for your enquiry to which I am glad to respond.
Unfortunately, STRATO do not offer the option to change the NS on the root domain, only on sub-domains.
I would therefore recommend that you do the following.
To create a sub-domain name, please login to your customer service area using your customer number and password.
Once you have successfully logged inside, please click onto the button with the "..." inside it and then select "Manage domains".
To the right of the domain name you would like to create the sub-domain for, please click onto "Manage" and then "Create sub-domain".
Here in the empty field, please enter www and save your settings by clicking onto "Create sub-domain".
Now you will need to setup the NS Records for your sub-domain, please click on "Manage" located to the right of the sub-domain, "DNS Management" and then "Manage" located to the right of "NS Record".
You will now be able to enter your desired NS record, and select "Accept settings" to save.
PLEASE NOTE: All DNS changes can take a full 24 hours before they are activated.
Last but not least, you will need to forward your root domain name to the sub-domain name.
Please select "Manage" located to the right of your root domain name. Inside this area, the first option will be "setup forwarding".
Now, you will be able to select the external forwarding. Please enter inside your sub-domain name. To save your settings, simply click onto "Enable forwarding" which is then active with immediate effect.
Should you require any further assistance, please feel free to contact me any time!
If you have 2 mins time for me, I would really appreciate it if you could complete my customer service questionnaire located underneath.
Many thanks in advance and have a great day!!"