If it has worked before but now reports it is overheating I'm not so sure the extension they offer will help.
There's nothing special about the Roku HDMI extension, it's just a short (4 inches or so) HDMI cord with an HDMI plug at one end and an HDMI socket at the other. I use one several feet long to locate my Streaming Stick+ in an equipment closet on the other side of the wall from my TV. You may be able to find an HDMI extension locally and not have to wait for someone to ship it to you.
I have not experienced an overheating problem myself, but here is some suppostion on my part: One thing that can change and I suspect will make your Roku work harder than before, can be a change in your local wifi environment. If neighboring wifi systems have started crowding the wifi channel your router is using, your Roku will have to sort through all the data packets it receives on that channel to find the ones coded for your IP. If you can configure your router to use a different channel that is less congested it might help.
If you are using the 2.4 GHz wifi band the channels that have the best chance of a relatively clean signal are 1, 6, and 11 as these channels don't overlap each other's frequencies, but depending on what channels are being used nearby, you could find your best results are on any channel 1-11. You can also reduce interference from adjacent channels if you can set the router to use 20 mHz Bandwidth instead of 40.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X