I do not recommend using VPN, it's better to ask duelbits what to do in this situation, you can contact their live support
VPN is fine with Duelbits if you don’t live on restricted country which is not the case on the user above that encounter the problem since he can access Duelbits using his smartphone.
I try to claim free spin many times on Duelbits during Christmas and Easter event with VPN without any problem. The support suggest it too since the provider of the free spin doesn’t allow me to play as long as I will just use that provider for claiming the free spin. I also have a default VPN on since I have premium subscription. VPN is only risky if you will use free VPN since they are using shared IP that might connect you to other user. Worst to user that violates the casino ToS.
A VPN is indeed fine but don't use a free VPN. I used Duelbits in the past a lot and accessed it sometimes with a VPN (I am not from a restricted country but need VPN's for other sites) and after a while it caused me issues and I can't participate anymore in any promotions. They never really explained which rules I broke (probably shared IP?) and even with doing a full KYC, I am still excluded from everything.
I don't think that there is any difference between a free VPN or Paid VPN if using for the purpose of gambling. The gambling sites allow the use of VPN's because they want more gamblers to gamble on their sites and they never blacklist any kind of VPN connections. You might have connected your VPN to a country where the promotions might be limited or you may have already availed most of the promotions. I don't think that shared IP is a problem for the gambling sites because they don't even care about IP's as far as I know. They want more and more users and that's the only truth, whether those users are connected via a shared IP or dedicated one they don't do anything against those users as long as they are placing bets on their site and aren't doing anything fishy.
You should try to contact their customer support team and ask them that why you are not eligible for the promotions and why they are not allowing you to be part of those promotions? I'm very sure that they will come up with a good reply and may solve your issue if that's a bug or a glitch of their system.