It seems that there are some known issues when the device uses DFS channels on 5GHz and has both, client and AP configured at the same time. If possible, could you please try changing the channel on the access point you are connecting to?
Regarding your question about the v7.5 FW. The current plan is to release it in July if everything goes according to plan. Unfortunately, I do not have any information regarding the issue with the captive portal, and we will have to wait for the release (I assume you are referring to the WiFi configuration in client mode when connecting to Hotspots).