I just double-checked on one of our servers and can confirm that at least Windows 2016 isn't using the ifX table (which you are trying to query). Considering SNMP on Windows has been deprecated for a while now, I wouldn't expect that to change. That also means using SNMP is probably not the best way to get this data from a Windows system in general.
For the ifTable, you'd want to query .1.3.6.1.2.1.2.2.1.10 for inbound and .1.3.6.1.2.1.2.2.1.16 for outbound. Again, I would not recommend doing this on busy Gigabit or faster connections as counters are going to overflow every few minutes and you won't be able to trust the data you're receiving.
There are two ways to setup Agents.
The original way is for the NetXMS server to connect to the client. This doesn't require any certificates at all, just a port forward for TCP 4700. If you only have one agent at the other end, this will work just fine for you.
The other way are Agent Tunnels, in which case the Agent will connect to the NetXMS server. This one requires certificates on the server (the server will then generate certificates for the clients as needed). There are some guides available on the forum and at least on Linux it looks more complicated than it really is.