You can connect the stb or other devices to the wan bridge and let them use their dhcp clients to obtain ip config on their own from the isp...
That was one of the first things I've thought about. But, if the other devices get their IPs from the provider, they will still be from the subnet, different from that of WAN bridge (if RB is connected to Internet via IPoE). So they are inaccessible from the RB itself and PC1 behind RB. Please correct me if I'm not right.
The other idea was to separately connect to the switch one more interface, e.g. ether2, not bridged with any other interface in the RB, and let it get the IP from ISP (like two WAN interfaces configuration). In this case the RB will be able to communicate with all other devices connected to the switch because they will be in the same subnet. But the switch is rather far from RB and to put another cable is not so convenient for me. I hoped that this could be done with some kind of virtual ethernet interface or something.
If there are no other ideas, I will prefer to stay with temporary second static IP for WAN, whenever I want to manage the switch.