no internet with prepaid O2 on Samsung dual sim

Hello!
It seems that the problem with dual sim smartphones internet connection via O2 prepaid simcards is quite frequent. And the ways of solving it are not so that clear.

In my case I purchased O2 Prepaid Sim 1.5GB LTE in Saturn 2 days ago, spent almost 3 hours to set it, but still have no Internet connection on my Samsung J2 Prime. At the same time this sim works well for phonecalls, and text messages. Of course, calls and text are fine, but they were not the reason for me of buying this tariff!

I've tried almost everything – switched sim cards between 1st and 2nd slot, inserted it into the other phone (where everything works fine), checked the APN settings and made some new profiles according to the info from the link below

https://hilfe.o2online.de/english-o2-community-43/o2-prepaid-sim-no-internet-193767

But nothing really works. Guys at Saturn were not able to help me, guys from O2 office on Alexanderplatz even refused to provide any service sending me to Saturn because "you bought your card there so you go there".

Please, provide me with any info that could help me fixing the problem. I've already had an experience with shitty Internet in Berlin, but this time it is something awfully new.

TBCMagic: Moved to Prepaid