It might be an issue with the router itself. You’ve already said that you’re using a fixed channel, so it should normally just work.
We’ve seen issues with Orange routers in the past, though you seem to be on Vodafone.
Take a look at this post. You should probably be able to modify the proposed script to meet your needs, without requiring ifupdown. Run it under root (sudo crontab -e) and don’t run it from /etc/rc.local, since that only works at at OSMC startup