Linksys WRT1200AC cannot do 2,4GHz and 5GHz in parallel

Hi, trying to use an WRT1200AC as mobile wifi-only accesspoint with a client and a master network enabled on both 2,4 and 5GHz. I can't get both radios working in parallel, the 5GHz does not start while 2,4GHz is active. Can anybody confirm this or is there any workaround? I have set all channels to auto and I have travel-mate packages installed. The channel must be auto because the client mode channel depends on the foreign upstream network.

Incorrect. You should set the channel on the upstream connection. OpenWrt does this during WWAN config.