Zabbix describes proxies for collecting data at remote sites, including operation through connectivity interruptions. That capability deserves a concrete test when evaluating the Zabbix alternatives page.
Use a disposable monitored host and interrupt only the proxy's upstream connection. Observe local collection, central visibility and the return of buffered data after reconnection. Keep historical catch-up separate from real-time alert delivery; measurements arriving later do not mean the central system saw the incident as it happened.
Akousa