Hey 
all
I was able to do my tax declaration very easily using wundertax.
I’ve contacted their customer support about my case Using non-german trading platform (Trading212) and they told me to simply declare the total capital gains in the last step in the tax declaration on the web app, just put it in the text-filed.
I did this and extracted my capital gains from the Trading212 report for the year 2020 and I’ve received my tax declaration assessment from the tax office based on the amount I’ve declared very smoothly with no problems 
I’m sharing here the template I’ve used to declare my capital gains.
English 
Version [For explanation]:
I would like to declare that I have made a profit of a total amount of 1000 Euro from selling stocks with a Foreign UK Brokerage (Trading 212 UK Ltd) for the year 2020. Unfortunately, Trading 212 UK Ltd does not provide a tax certificate by the end of the year like the German brokerages and does not deduct any taxes from me.
German 
Version [I’ve used]:
Ich möchte hiermit erklären, dass ich für das Jahr 2020 einen Gewinn in Höhe von insgesamt 1000 Euro aus dem Verkauf von Aktien bei einem Foreign UK Brokerage (Trading 212 UK Ltd) erzielt habe. Leider stellt Trading 212 UK Ltd zum Jahresende keine Steuerbescheinigung wie die deutschen Broker aus und zieht von mir keine Steuern ab.
Hope this helps everyone