Two possible issues here:
1. Does the ADSL router have a static route to 10.16.1.0/24 network? If not, try adding that and see if that fixes it. If it doesn't, go to issue #2.
2. If NAT isn't setup properly, then your "inside" hosts won't be able to get to the internet. If you think about how NAT works, it makes sense. For example, let's say you have two hosts to make it easy. Host 1 is 10.16.1.3 and Host 2 is 10.16.1.4. The Gateway address is 10.16.1.1. Your server should have a route for 0.0.0.0 0.0.0.0 192.168.1.2. All your clients will in turn send their traffic to their default gateway of 10.16.1.1. When traffic arrives there, to a destination the server doesn't have a route for, it uses it's WAN IP to forward the traffic out; 192.168.1.2
You can verify what routes the server is using either through RRAS or by running the command, "route print" from a command prompt.
If the route exists, and is being used (i.e., has the lowest metric) then your issue is probably with NAT. When either of your clients sens data to the internet, it goes from 10.16.1.1 to 192.168.1.2 on your ADSL router, to your ISPs routers, and then to the internet. When the host out on the internet responds, it goes to your ISPs routers, then your ADSL router's address, which then shoots it to the 10.10.16.1 address. This is where the problem happens. Your server doesn't know which client to forward the packet to from there because it's not utilizing a NAT table to keep track. Is it the 1.3 or 1.4 client?
So basically the packet is most likely stopping at 10.16.1.1 when it comes back FROM the internet host.
Traffic is going from 1.3 or 1.4>10.16.1.1>192.168.1.2>your ISP>Internet Routers>Internet Host>Internet Routers>ISP>ADSL router>10.16.1.1>stuck.
edit: It's been a while since I've messed with routing and NAT so you might want to double check my IPs and such