Gigantothermy (sometimes called ectothermic homeothermy or inertial homeothermy) is a phenomenon with significance in biology and paleontology, whereby large, bulky ectothermic animals are more easily able to maintain a constant, relatively high body temperature than smaller animals by virtue of their smaller surface area to volume ratio.[1] A bigger animal has proportionately less of its body close to the outside environment than a smaller animal of otherwise similar shape, and so it gains heat from, or loses heat to, the environment much more slowly.
Great white sharks are an example of gigantotherms. The sharks can warm up their core by shunting blood past the working muscles (which generate heat) and from there into the core, where their large mass limits heat loss to the environment. https://en.wikipedia.org/wiki/Great_white_shark#Adaptations
Other "cold blooded" animals have similar workaround that let them stay warm - bluefin tuna are another example.
Your reptile should be massive. It will need to stay moving, generating heat from its muscles. Reptiles generally do not need to eat as much as mammals but under these circumstances your reptile sentient will need to eat a lot to provide energy which it will use to move and generate heat.
It might also shiver a lot; that works for us and would work for it too, but make it a terrible shot. If is is called upon to do any detail work it might need to let itself cool down. If it cools down too much it will be stuck and have to wait to be warmed by external heat.
Why would a reptile sentient from a hot planet need this trick? Maybe, like the shark and tuna, it is semiaquatic and the water can be cold. Maintaining body heat in the water let its ancestors hunt.