We don’t have enough information to go on…
– what are the netmasks in use
– is there a routing protocol in use here, or using purely statics
– details on statics, or routing protocol being used?
Possibly, you can see the 192. side of the router as it looks like just another device on the network; but hasn’t been set up to do forwarding TO the 152. side; and the 152 side needs to know the way back; although in this scenario with no downstream networks in place (??) that return packets would work IF the downstream lab devices at least had their gateways set to 188.8.131.52.
As stated above: we need a bit more info to be 100% accurate.
However, you did not mention
1) if you were using any routing protocols (RIP, OSPF, EIGRP) for routing on the 2811
2) If you have static / return routes on the 2811 back to the 6509
Based solely on what is provided, best place to look is routing on the 2811…