This is a known issue.
RnD is working on a solution at the moment, but it is planned to be included only in 7.4 firmware release.
Could you login to the device via SSH and test if any of the commands below restarts WiFi interface:
- /sbin/wifi reload
- /etc/init.d/network restart
If any of them works, it could be included in the startup script in System -> Custom scripts section, to automate the restarts.
Otherwise, It is suggested to stay with a working firmware version for a while, if that does not create any inconveniences to you.