Older Yamaha synths had no defined reset procedure, just a requirement to re-load all the sounds and deliberately re-change all the options (as sometimes they can attain corrupted/illegal values and cause strange behaviour). I did this when I bought a supposedly broken DX7: it was claimed faulty but in reality, seemed merely to have had a bump and temporarily lost power from its battery – which held the re-filled RAM for weeks until I replaced it with a holder.
Course, if the DX21 does have a procedure, a search should find it for you, as it would also verify the battery type.
id keep the battery in place with elctrical tape if youre putting it in a holder, so it wont get loose or fall out accidentally when moving the instrument
I would tape the top of it in any case. I had a TX81Z that seemed unable to hold its memory despite various fixes to the (somewhat damaged) battery circuit, until I realised that the battery was simply shorting against the metal case lid and dropping the memory for that reason. When it came time to put a holder in my V50, I had to do the same thing – as the bottom of the case ended up pressing on the battery, which I had cheekily mounted on the solder side of the PCB, in order to make hypothetical future replacements a lot easier.