I was using CLAT in an Archer C7 v5 for some testing. Everything worked fine.
I decided then to upgrade to 19.07.1 and everything works fine from the backup that I did from the config, but not the CLAT (464XLAT).
I decided to reflash the unit from scratch and configure everything manually. Same problem, CLAT is not working. I tried even setting the tunnel link to the WAN interface manually in the CLAT interface (advanced settings), instead of the default "unspecified", played with the firewall settings as well, etc. Nothing resulted.
By the way, even in 18.06.7, which I'm using right now with the CLAT, the CLAT interface doesn't report any RX/TX or packets. In the same router I've installed Collectd and I can see there the traffic being graphed correctly.
So I'm guessing there is some bug there. I'm going to report it as a bug, but just in case someone discovered an easy solution.