Remotely accessing hundreds of systems from head office

Hi Everyone. I’m looking for some ideas regarding a secure solution for a small company I’m working with.

We are looking to deploy 20 remote systems that need to be secured as they share information with the cloud and head office. In the future, we are looking to scale this up to roughly 200 remote systems (1-2 years). These remote systems have a computing device running CentOS and various sensors (~20 misc sensors) connected to the computing device or a switch depending on their medium.

How do I set these first 20 deployments so that I can remotely access the computing device and all sensors (individually ideally - because some of them have remote software for firmware, configuration, etc.) from head office? Ideally, I'd like to host a VM in the cloud with the sensor IO management software so that all techns could utilize the system without installing software on everyone's PCs.

Here are some of the things I looked into:

1. Our Routers (with LTE) are only capable of being OpenVPN Clients, does this mean that I cannot remote into their specific network and view all devices within that LAN?
2. Could I host the OpenVPN Server on the computing device within each network and forward the router traffic to the computing device before anything? (I'm guessing this will require serious computational power?)
3. Could I host a OpenVPN Server in AWS, Azure, etc. and point all devices to go there per remote system?
4. The routers we have are capable of IPSec tunnels, would that work in place of the VPN solution because the router can’t be a VPN server?

Thanks for your help!