@chrishamm Thanks so much for getting back to me. I don't think it is an ISP issue since I am able to download the packages on my PC, which is on the same network. Further to this, I have had this issue for about 4 months now and only gotten around to solving this issue this week.

I hope anyone in this form can help me. I am currently on version 3.4.6 but I would like to upgrade to the latest version. Duet is setup on a SBC, on a Raspberry Pi 5 and works otherwise flawlessly. When running the commands sudo apt update and sudo apt upgrade I get the following response

W: Failed to fetch https://pkg.duet3d.com/dists/stable/InRelease  Could not connect to pkg.duet3d.com:443 (134.209.94.168), connection timed out
W: Some index files failed to download. They have been ignored, or old ones used instead.

Ign:1 https://pkg.duet3d.com stable/armv7 arm64 duetsoftwareframework arm64 3.5.0
Ign:2 https://pkg.duet3d.com stable/armv7 arm64 duettools arm64 3.5.0
Ign:3 https://pkg.duet3d.com stable/armv7 arm64 duetpluginservice arm64 3.5.0
Ign:4 https://pkg.duet3d.com stable/armv7 arm64 duetcontrolserver arm64 3.5.0
Ign:5 https://pkg.duet3d.com stable/armv7 arm64 duetruntime arm64 3.5.0
Ign:6 https://pkg.duet3d.com stable/armv7 arm64 duetwebserver arm64 3.5.0
Ign:7 https://pkg.duet3d.com stable/armv7 arm64 duetwebcontrol all 3.5.0
Ign:8 https://pkg.duet3d.com stable/armv7 arm64 reprapfirmware all 3.5.0-1
Ign:1 https://pkg.duet3d.com stable/armv7 arm64 duetsoftwareframework arm64 3.5.0
Ign:2 https://pkg.duet3d.com stable/armv7 arm64 duettools arm64 3.5.0
Ign:3 https://pkg.duet3d.com stable/armv7 arm64 duetpluginservice arm64 3.5.0
Ign:4 https://pkg.duet3d.com stable/armv7 arm64 duetcontrolserver arm64 3.5.0
Ign:5 https://pkg.duet3d.com stable/armv7 arm64 duetruntime arm64 3.5.0
Ign:6 https://pkg.duet3d.com stable/armv7 arm64 duetwebserver arm64 3.5.0
Ign:7 https://pkg.duet3d.com stable/armv7 arm64 duetwebcontrol all 3.5.0
Ign:8 https://pkg.duet3d.com stable/armv7 arm64 reprapfirmware all 3.5.0-1
Ign:1 https://pkg.duet3d.com stable/armv7 arm64 duetsoftwareframework arm64 3.5.0
Ign:2 https://pkg.duet3d.com stable/armv7 arm64 duettools arm64 3.5.0
Ign:3 https://pkg.duet3d.com stable/armv7 arm64 duetpluginservice arm64 3.5.0
Ign:4 https://pkg.duet3d.com stable/armv7 arm64 duetcontrolserver arm64 3.5.0
Ign:5 https://pkg.duet3d.com stable/armv7 arm64 duetruntime arm64 3.5.0
Ign:6 https://pkg.duet3d.com stable/armv7 arm64 duetwebserver arm64 3.5.0
Ign:7 https://pkg.duet3d.com stable/armv7 arm64 duetwebcontrol all 3.5.0
Ign:8 https://pkg.duet3d.com stable/armv7 arm64 reprapfirmware all 3.5.0-1
Err:1 https://pkg.duet3d.com stable/armv7 arm64 duetsoftwareframework arm64 3.5.0
  Could not connect to pkg.duet3d.com:443 (134.209.94.168), connection timed out
Err:2 https://pkg.duet3d.com stable/armv7 arm64 duettools arm64 3.5.0
  Unable to connect to pkg.duet3d.com:https:
Err:3 https://pkg.duet3d.com stable/armv7 arm64 duetpluginservice arm64 3.5.0
  Unable to connect to pkg.duet3d.com:https:
Err:4 https://pkg.duet3d.com stable/armv7 arm64 duetcontrolserver arm64 3.5.0
  Unable to connect to pkg.duet3d.com:https:
Err:5 https://pkg.duet3d.com stable/armv7 arm64 duetruntime arm64 3.5.0
  Unable to connect to pkg.duet3d.com:https:
Err:6 https://pkg.duet3d.com stable/armv7 arm64 duetwebserver arm64 3.5.0
  Unable to connect to pkg.duet3d.com:https:
Err:7 https://pkg.duet3d.com stable/armv7 arm64 duetwebcontrol all 3.5.0
  Unable to connect to pkg.duet3d.com:https:
Ign:8 https://pkg.duet3d.com stable/armv7 arm64 reprapfirmware all 3.5.0-1
Err:8 https://pkg.duet3d.com stable/armv7 arm64 reprapfirmware all 3.5.0-1
  Unable to connect to pkg.duet3d.com:https:
E: Failed to fetch https://pkg.duet3d.com/dists/stable/armv7/binary-arm64/duetsoftwareframework_3.5.0_arm64.deb  Could not connect to pkg.duet3d.com:443 (134.209.94.168), connection timed out
E: Failed to fetch https://pkg.duet3d.com/dists/stable/armv7/binary-arm64/duettools_3.5.0_arm64.deb  Unable to connect to pkg.duet3d.com:https:
E: Failed to fetch https://pkg.duet3d.com/dists/stable/armv7/binary-arm64/duetpluginservice_3.5.0_arm64.deb  Unable to connect to pkg.duet3d.com:https:
E: Failed to fetch https://pkg.duet3d.com/dists/stable/armv7/binary-arm64/duetcontrolserver_3.5.0_arm64.deb  Unable to connect to pkg.duet3d.com:https:
E: Failed to fetch https://pkg.duet3d.com/dists/stable/armv7/binary-arm64/duetruntime_3.5.0_arm64.deb  Unable to connect to pkg.duet3d.com:https:
E: Failed to fetch https://pkg.duet3d.com/dists/stable/armv7/binary-arm64/duetwebserver_3.5.0_arm64.deb  Unable to connect to pkg.duet3d.com:https:
E: Failed to fetch https://pkg.duet3d.com/dists/stable/armv7/binary-arm64/duetwebcontrol_3.5.0_all.deb  Unable to connect to pkg.duet3d.com:https:
E: Failed to fetch https://pkg.duet3d.com/dists/stable/armv7/binary-armhf/reprapfirmware_3.5.0-1_all.deb  Unable to connect to pkg.duet3d.com:https:

The SBC has access to the internet, which was verified by ping google.com and also ping www.duet3d.com for both commands. I get a response, but for some strange reason, not for ping pkg.duet3d.com. Further to this when I run sudo apt update && sudo apt upgrade -y it updates and upgrades all other packages.

I have gone through my installation, and I cannot find the reason why it refuses to connect to the duet3d server. But there is clearly something not quite right.

Thank you very much already for your help.