My Nas326 http port alway auto close

I alway connect to my nas , from out side home network
my router is support for UPnP Function , I also already do Port mapping http port

Problem is sometime http port in UPnP function is auto close,
but my public ip isn't change 

at first i think problem is from my ISP router , but i also purchase a new isp but still got a problem

then, i search for more in formation Some tell me Problem with my NAS ram (326 have 512MB)
then i close all fuction i didn't use , but the same still got a same problem 

now i try to use a tweak ( From @Mijzelf answer ) 
i want to create SCRIPT that alway reset my http port , every 1 hour
can someone help me 

sorry for my bad eng
thank for all reply



#NAS_Oct_2019