I have a small USB fan that I plug into my power bank between batteries. I also use the power bank to power my WiFi extender.
I have another toy drone (not nearly as much fun as my Tello) but it heats up pretty good too. That drones manual says to let the drone motors and circuit board to “rest” (cool down) for 10 minutes otherwise you shorten the motor life. Sounds like good advise to me so I do the same between Tello batteries.
I do not have that problem. In fact last week I flow two days using my three bateries, just one after other without any rest. After the 30 minutes of continus flight I touch the motors to check if they are hot and they are just a bit warm. In my case flights were indoor and amb. temperature of approximately 25 ºC.
In my case I am outside at 95F (35C). The Tello Manual says the Operating Temperature Range is a maximum of 104F (40C). It doesn’t overhead in flight because the props act as a fan, but as soon as I land at any time after a few minutes it shuts itself down due to “overheating”. I flown the battery down to 10-15% and it does not overheat in flight. That is good.