Remote management has always two challenges:
* connectivity regardless of network configuration
* remote management software either depending on network configuration or not

VPN on demand
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Hamachi - proprietary - up to 5 connections for free
https://www.vpn.net/
https://help.ubuntu.com/community/Hamachi
GUI for Hamachi (FLOSS) - https://www.haguichi.net/

Hamachi FLOSS alternatives http://alternativeto.net/software/hamachi/?license=opensource

Others:
https://www.zerotier.com/
http://alternativeto.net/software/zerotier-one/?license=opensource

https://wormhole.network/

# # #

Remote management
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NX - proprietary
https://www.nomachine.com/
- depends on network configuration
- has no limits for up to 5 connections
https://en.wikipedia.org/wiki/NX_technology

NX FLOSS alternatives http://alternativeto.net/software/nomachine/?license=opensource

NX log in using SSH key file
https://www.nomachine.com/AR02L00785
cat ~/.ssh/key.pub >> ~/.nx/config/authorized.crt

NX3 protocol forks:
https://www.nomachine.com/version-3
https://help.ubuntu.com/community/FreeNX
https://code.google.com/archive/p/neatx/
http://wiki.x2go.org/
http://xpra.org/

* * *

Teamviewer
https://www.teamviewer.com/
- does not depend on network configuration
- proprietary
- interrupts connection for 2 minutes after each 5 minutes

Teamviewer FLOSS alternatives http://alternativeto.net/software/teamviewer/?license=opensource