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So, the server is a Windows Server 2012 with a nat and internal network card.
The guest is a Windows 7 client with an internal network card.

Is it possible to share the internet connection through internal network?

I've already tried configuring Remote access - > routing on the server but I can't make it work so far. I'm not sure if it's even possible this way

If Server 2012 has NAT then it would highly likely have internet. The internal network does not connect to anything except the Virtualbox guests attached to it. If the Server 2012 has internet connection sharing then 2012 can probably pass internet through itself to the internal network. See 2012's help files & google results to find out if internet connection sharing is a feature of 2012.

If 2012 does not have internet connection sharing or you don't want to use it, you will have to add another guest running a router OS like pfSense, with the internal network on the router OS's LAN side, and NAT or Bridged to the WAN side.

I don't know how you failed to set up RRAS as a NAT router. It is one of its basic functions and has an option for it in the setup wizard. What did you select as the private and public interfaces in the wizard?

I selected the NAT for public and internal for private, obviously.

Yes it is obvious, but the only thing I can think of which could possibly be wrong. Have you rebooted the server?

What doesn't work? Does the guest get its network config from NAT on the RRAS server? Can you ping your gateway router from the vm?

So, the server has a NAT and an internal card, the guest has an internal card only.
I installed DNS and DHCP on the server and made fix ip for both cards.
Then installed RRAS and selected the external card.

I can ping the server from the clients but can't ping the card I selected at RRAS.

Don't be distracted by the fact that you're running your guests in Virtualbox, then come to the conclusion that it's a Virtualbox problem 100%. If your server guest could get internet, and could access the W7 guest, before adding all this RRAS stuff, then Virtualbox is doing its job setting up network infrastructure properly, and you have to look to Windows sources to figure out RRAS.

Just do a Google/Bing search on Windows 2012 and RRAS, and you'll see that plenty of folks trying this on physical PCs have questions and issues. Forget that Virtualbox is hosting the guests, and look for solutions as if you were running physical PCs connected together, and you should be OK.

Yeah, the problem is me, not knowing crap about this topic Is there anything I have to do on the clientVM after sharing the net on the server? I just can't see anything on the client, just the internal card which doesn't connect to the internet

Honestly, it beats me. Never tried setting up a server as a router. (I have successfully set up pfSense as a guest router for a Virtualbox network.)

Be sure that your server guest's internal network adapter and the Win7 guest's internal network adapter are both set to the same internal network name. Your Win7 guest should be looking to DHCP for an IP address, which it should get from the 2012 guest. Further setup is beyond what Virtualbox can influence and requires googling Windows help sources.

The client gets its ip from the server DHCP, can ping back and forth, internet on the client still not working :/

I would look at the gateway then. RRAS should be setup in a way to pass the proper addressing scheme to each domain guest. That is what the first R stands for ( routing ).