It's always a good idea to give your router and Fibre box a restart. Sometimes cache and temporary files can build up, slowing them down.
Look for your fibre box on the wall, simply turn the power off at the wall and turn it back on.
If you have a WiFi Hub, you can restart it by pushing the black button on the back of the router to turn off the power, then simply press it again to turn the power back on. It may take a minute or two to turn back on.
Once your fibre box and your router has been switched off and turned back on, re-try what you were doing before.
Has this resolved your issues?