The syntax of the message is 100% ok: it's possible to use RNG segment in conjunction with TMP segment.
Anyway I'm going to quote, here below, Recommendation JM4/239 from ITIGG documentation:
The TMP should always be sent where temperature-sensitive cargo is carried in an item of equipment. If a temperature range is to be specified, the TMP should be used in conjunction with the RNG segment - in this case only DE 6245 of the TMP should be used, and the temperature range should be specified in the RNG. If a single temperature setting is to be specified, C239 in the TMP should be used.
That's it ! Of course that's only a recommendation but I fully agree.