AC-68U with Shibby v121: Bandwidth limiter not limiting upload speed [conflict with Captive Portal]

I'm trying to limit bandwidth for a guest network on br1. While download speed limit works fine, upload limit doesn't seem to have any effect at all: I'm getting the same upload speed test results as on my private network. I read somewhere on the net that creating a virtual lan for that bridge fixes it, but it didn't help in my case.

Here are some relevant screenshots:
View attachment 3124 View attachment 3125

Any help is greatly appreciated.
Alex