Routing between two traffic group

You can either disable strict isolation on both route domains or use the parent-child relationship between route domains. While this resolves the problem, it may break your business logic for which you implemented route domains, so depending on your needs it may be better to use an external routing node.  

Quoting the doc:

About parent IDs:

When you create a route domain, you can specify the ID of another route domain as the parent route domain. The parent ID identifies another route domain that the system can search to find a route if the system cannot find the route within the child route domain.

About strict isolation:

You can control the forwarding of traffic across route domain boundaries by configuring the strict isolation feature of a route domain:

• If strict isolation is enabled, the BIG-IP® system allows traffic forwarding from that route domain to the specified parent route domain only. This is the default behavior. Note that for successful isolation, you must enable the strict isolation feature on both the child and the parent route domains.
• If strict isolation is disabled, the BIG-IP system allows traffic forwarding from that route domain to any route domain on the system, without the need to define a parent-child relationship between route domains. Note that in this case, for successful forwarding, you must disable the strict isolation feature on both the forwarding route domain and the target route domain (that is, the route domain to which the traffic is being forwarded).