as far as i know you can register from any country, but if you have any problems opening the site, which i did, you can just use a VPN.
I wouldn't recommend doing that! At least, contact customer service and ask them if it is OK to use VPN to access the casino or wait for their representative to give you the right answer. If the use of VPNs is prohibited then you mace troubles later when you want to withdraw your money.
I checked their terms of service and didn't find Austria on the list of restricted countries so not sure why it doesn't show on the drop-down list?
Maybe the casino doesn't limit the country but the country blocks the site so it's difficult to access, as I experienced in my country gambling is very illegal but we can still access for several private providers except for government providers it's definitely not going to be able to access the site, so I use a provider private sector and can still access casinos before I couldn't really access casino sites using genuine government providers so using a VPN was not the right way.
It is true that it is better to talk to the CS on the casino site whether it is safe to use a VPN, because there are many cases where the account may be frozen because the system detects fraud, so it must be discussed with the casino, before you try to access it using a VPN and gamble there without knowledge.