Generally, when a phone says there is no update available, it is usually true.
I can find very little information on the HOXL+, but it is likely safe to assume that there are no updates released for it if it tells you so.
If you really want to update, you could root it and flash a custom rom on it, I do not know if the HOXL+ is one-to-one compatible with the HOX+ though. If it is, you can flash any HOX+ rom on it. If it is not, well, I haven't seen a HOXL+ sub forum on xda yet, I think you're out of luck then.
If you are in doubt about the compatibility with the HOX+, I suggest staying on whatever you are currently running, as reverting back to a working phone is going to be hard if it turns out not to be compatible.
EDIT: Your phone seems to identify itself as Envitareul, whichis the codename of the AT&T version of the HTC One X+, not the international version. This does not mean per se that it is the same device, or that it is compatible, but maybe the AT&T HOX+ folks on the forums can tell you more about that.