Need help with ZYXEL (PMG5617-R20B) unable to open ports.

Sorry if I post my query in wrong section. 
I am an Airtel Customer in India. My ISP installs new router ZYXEL (PMG5617-R20B). The options to open ports are not like other routers in this router. so I visit here for help. 
My ISP is unable to open ports on this router. I tried to set the port trigger options, but it open ports temporary. The port auto closes and open randomly. Tested with online open port checking websites sometimes the port is working fine and sometimes the port shows as closed. 
Please help.
Need some router manual or setting to open port on this router. Please help us. 

Thank You. 

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