What monitoring system to choose for servers behind NAT?

Question

Monitoring

Good afternoon.
Please tell me which monitoring system(s) to choose for the following situation:

• we have several dedicated servers and vps (up to 10) in the local network and in data centers.
  
• those servers that are not accessible from the outside in the local network (there is no way to forward the port), i.e. the monitoring system needs an agent that will send data to the central server itself.
  
• data can be collected both on my vps and on third-party servers
  
• the system should be free.
  
• it is desirable that you can put an application on your smartphone to view data about monitored servers.
  

There are newrelic and zabbix in mind.
What do you advise? The choice is simply huge, it is difficult to choose.