So I was having problems getting my 600EX II-RT speedlites to work with my Westcott FJ-X3M trigger (I also own and use Canon's Speedlite Transmitter ST-E3-RT (Ver.2)). I finally solved my problem so I'm posting this to maybe help others. So for Canon's transmitter, it's already using Canon RT as the protocol. On the Westcott, you have the option to use Canon RT or Westcott's protocol which they simply labeled "Canon." So if you're strictly using Westcott lights, you should use Canon, but if you're mixing in Canon speedlites, use Canon RT for everything (the Westcott lights can be set to using Canon RT). Set up the speedlites and strobes to have the same channel and same ID number. So for example, I set channel 12 for my Westcott FJ400 lights as well as my Canon 600EX II-RT speedlights. I set the channel ID in the Canon speedlites to 0001 whereas I set my Westcott trigger and lights to 01. I set my Westcott lights to group A. I set my Canon speedlites to group B to be used as hair lights.
Now here's the key. Once you have set the settings, restart all devices. That means powercycling (turn off and back on) the strobes, the speedlites, and the trigger. When they restart, they will get the link. Now, here are 2 keys. The first key is when the camera goes to sleep, you can lose the link. The only recovery is to powercycle all the devices again to reconnect the link. So for your sanity, you want to lengthen the time it takes for the camera to go to sleep. By default, my Canon R5ii is set to 3 minutes to going to sleep. However during studio sessions, I set it to 30 minutes. The next key is if you change the settings for the wireless connection (i.e., changing the ID or the Channel number). For reliability, if you change any of the wireless settings, you have to powercycle all the devices again. You're allowed to change the power setting of the speedlite (for example, going from 1/128 power to 1/64 power) without powercycling. But if you change the wireless connection, you will need to powercycle.
As long as the camera doesn't go to sleep, you will maintain the link. I've tried this with my Westcott trigger as well as the Canon Speedlite Transmitter ST-E3-RT (Ver.2) and was able to maintain long connections. Good luck to the frustrated!
