Joiplay had RTPs downloadable into their app back then but since it buggy af, they removed it. now, there's 2 ways to doing it:
find an rar/zip file of extracted VX/VX ace (depend on what game you'll played, if it has RGSS301.dll on its folder, then is VX ace. if its RGSS201.dll, its just VX) RTP on google, extract it on your phone, open your game on Joiplay and choose that extracted folder.
OR
download free RTP installer from official RPGmaker site, install it on a PC (I suggest not install it on drive C), copy it into your phone, open your game on Joiplay and choose that extracted folder.