I could not book it via rw.by so I had my friends in Russia book it for me on the RZD website. If you know anyone in Russia I recommend you do this as the russiantrains website grossly overcharged.
Rw.by was good for tickets between Russia and Belarus and domestic Russian trains.
Another option of you have a VPN with Russian virtual IPs like Proton, is to get set up with Pay In Russia and their 10% fee to get rubles will be less than the 30% that these agents charge. Then you can use those rubles to book ok RZD.