MA5680 can access the upstream server through the optical modem, but not vice versa.

I did an experiment and found a depressed problem. 

Computer------->Optical Cat------->MA5680------->PPPOE server This is normal, and the dial-up can be successful.

But why doesn't it work if I change the location of the PPPOE server and the computer?

Computer-------〉MA5680--------〉Light modem-----〉PPPOE server computer dialing error 651, and the IP on the server cannot be obtained, but the computer can be set to a fixed IP and can ping Through PPPOE service.

Could there be any security strategy on the MA5680 that is blocking it and it cannot be reversed? 

Thanks