@RobertWa 

you need to create a new VPN policy exactly the same as the one you have, but with the new remote subnet.

One VPN profiles for each remote subnet.

If you use IKEv2 you can add several remote subnets but this I have never been able to work against Cisco only with another ER7206 or ER605

So I have to do the same with IKEv2 between ERxxx and Cisco Firewals, other brand have I not tested with Site to Site VPN.

And you have very old firmware on your router, try to upgrade then you have more VPN option, 

Thanks @shberge! IKEv2 is def necessary for multiple subnets. But even after firmware upgrade I was not able to establish connection. But I am working against a Fortigate 1500 D, maybe that's the issue.

Thank you  @Hank21 - With Omada Controller I was able to add multiple subnets.

  @RobertWa 

it is probably the same as I struggled with against Cisco firewall,

I have to create a VPN profile for each subnet to make it work, so if you have two remote subnets you need two VPN profiles on ER7206

it is the same on IKEv1 and IKEv2, although you can define several subnets on IKEv2 it does not work against Cisco.

On Cisco Firewall, I only need one VPN profile

There are a lot of VPN bugs on TP-Link but it works if you do it this way, at least against the Cisco firewall.

I have not tested against anything else so I do not know if it is the same on your firewall

  @Hank21 Could it be, that there is a limit of max 5 remote Subnets? 

That´s a problem ! 

BR 

Andy