First, double-check to make sure your connection information is correct in InfrastructureAI (under My Equipment > [your gateway] > [your piece of equipment] > Edit Icon). Make sure you haven't fat-fingered your IP address. If you're using SNMP v2c or v3, make sure you know your community string and have it correctly entered. If you're using Modbus, make sure your Device ID is correct.
If you've setup your equipment correctly and your InfrastructureAI gateway still can't see the equipment, there's a good chance that it has been locked down to only report to certain IP addresses (also known as a whitelist). This is a security feature often seen on Liebert/Vertiv equipment and on equipment from other major manufacturers.
You might be wondering "I can see the equipment from my computer, so it's not locked down, right?" Well, actually, you are viewing your equipment through the HTTP protocol (used by web browsers). SNMP and Modbus are a different story and may be significantly more locked down by default because of their controllable nature.
To allow access to an InfrastructureAI Gateway from a Liebert/Vertiv Unity Card, see Configuring Vertiv UNITY-DP Cards to connect via SNMP. A similar process is used for Modbus, using the "Trusted IP List" on a Unity Card.