I recently bought Yongnuo YN600EX-RT II. I wanted to be able to use it off camera so due to recommendation bought YN622C-TX wireless flash controller. I cannot however get the flash to pick up the controller. I have set the flash to wireless slave mode and made sure that they are both on the same channel - beyond that I din't know what to do
Sorry, but you were given a bad recommendation. The YN622C-TX is not compatible with the YN600EX-RT II. See if you can return it, and buy the Yongnuo YN-E3-RT which is fully compatible with your YN600EX-RT II.
Yongnuo actually has three separate systems:
1) The first was a simple trigger system called RF-602, RF-603, RF-605 and was later upgraded to include remote manual power adjustment with the YN560-TX transmitter and YN560 III, IV and YN660 with built in receivers.
2) Next was the YN622 system wihich is a TTL system that is designed to operate with the YN622 receivers and the YN685 flash, but also has some limited backward compatibility with the YN560 system.
3) The YN600EX-RT is a clone of Canon's RT system and is not directly compatible with either of the first two systems.
The only way to control a YN600EX-RT II using a YN622C-TX is to attach a YN622 compatible receiver to the hot foot of the flash and set the flash as if it were mounted on a camera's hot shoe. The YN622C or YN622C II receiver will control the flash as if it were the camera's hot shoe. The radio receiver inside the YN600EX-RT II uses the Canon RT wireless radio protocol. This is a different "language" from the wireless radio protocol used by the YN622 system.
The YN-600EX-RT II's built in radio trigger is incompatible with the YN-622-TX radio transmitter.
Yongnuo has three separate, mostly incompatible radio triggering systems:
- The 560/60x manual only triggers
- The TTL/HSS 622 triggers
- The Canon RT clone gear
Yongnuo really makes it a PITA to figure out what works with what and you are far from being the first person to make the error of assuming all Yongnuo flash gear would work together. (See also: What are the Yongnuo flash naming conventions?)
You can distinguish which Yongnuo gear is compatible with the YN-600EX-RT II by looking for RT in the device name. You need a transmitter that works in the Canon RT system, although it's probably best to stick to the Yongnuo YN-E3-RT or the Canon ST-E3-RT, since there are reports the Jinbei RT-clone system isn't compatible. Not sure about the Phottix Laso triggers.
So, you could swap your YN-622-TX for a YN-E3-RT. You could add a YN-622 transceiver to the foot of the YN-600EX-RT II to act as a radio receiver and use it with your YN-622-TX. Or you could swap the YN-600EX-RT II for a YN-685 which has a built-in YN-622 receiver in it and would work with your YN-622-TX.
You may want to consider looking at other flash systems where there's a single integrated radio system that lets manual and TTL gear work together. Godox's X system is very popular these days, because their gear is priced in the same neighborhood as Yongnuo's, everything's in the same triggering system, Godox supports mirrorless systems as well as dSLRs, and has bigger lights for future expansion. But if you really want compatibility with Canon's RT radio system, Jinbei's and Phottix's RT gear does that.