Hey LeoAus,
Thanks for the update. Since the issue is resolved for a bit after restarting the router and since only download speed is affected, it may be a setting issue. Try forgetting the network and connecting to it again and see if the issue persists. Then try resetting the network settings and see if the issue persists.
Are there connection speed issues with other Wi-Fi networks?
Back up iPhone
- Go to Settings > Wi-Fi.
- Tap next to the Wi-Fi network that you want your device to forget.
- Tap Forget This Network, then tap Forget to confirm."
How to forget a Wi-Fi network on your iPhone, iPad, iPod touch, or Mac
"Reset your Network Settings. Tap Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you’ve used before."
"Update your Wi-Fi router with the latest firmware and make sure that the router supports your Apple product. For more information, contact the router's manufacturer."
If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network
Restore your iPhone, iPad, or iPod to factory settings
Have a good one.