Internet connection dropped when nat created and routed via vpn [linux+iptables]

Question

I've got a very frustrating network issue. Currently I'm living in a hotel in which I need to authenticate through a website in order to obtain internet access. I've also got xbox360 that I want to connect to the internet. As I cannot open the web browser on xbox until I am already online (sic!), I've nated the connection. Thus, my laptop works as a router and allows in/out traffic to xbox. Xbox sends requests to dhcpd active on my computer and is automatically assigned an address (always the same). So far everything works fine.

However, to make it more complicated, I've got regional issues when downloading games with gold. It implies that I need to change my IP if I want to use the service. This is where I would normally use vpn and cut it short. Unfortunatelly, life is not as easy. I can establish a connection between my laptop and vpn provider (openconnect) and it is persistent - no issues occur. However, when I connect my xbox and put it behind nat, the connection either breaks instantly or lasts 1-2 minutes and breaks. This is exactly the same configuration I'm using with the only difference that the traffic is router via tun and not eth anymore.

It is also worth mentioning that I cannot authenticate again unless I disconnect the ethernet cable. Then, I still get a couple of disconnects and the situation stabilises eventually.

The addressing would be the following: wlan (connecting to gateway): 10.x.x.x; eth (xbox connection): 192.x.x.x; tun (vpn tunnel): 172.x.x.x.

The router serving the gateway and authentication is Mikrotik. I did not see the symptoms of checking for nat and disabling the connection (as the connection without vpn works perfectly fine), but I've tried the following, nonetheless:

1) incrementing ttl; 2) spooofing wlan card address to eth; 3) a couple of other things more or less important.

I might also add that the internet connection is not broken when e.g. laptop is turned off. Once it is started, there is 12 hours limit after which I need to relogin. That would exclude any issues with 'ping-ponging' (which, even if it was the case, should not be a problem).

This has been really frustrating for me, does anyone possibly has any idea about what the disconnections might be casued by and how to prevent them?