How to enable DMZ on HiLink modems with crippled UI (ie. PlusGSM), to enable public IP access
(if you paid for it in the first place of course :) )

curl -X GET http://192.168.8.1/api/webserver/SesTokInfo

The response will be similar to the following:
<?xml version="1.0" encoding="UTF-8"?>
<response>
<SesInfo>SessionID=n2YhsMuKk1RgoEyoQO8QFXmCtDU0523wHFgB9fNa0otYlEtBgAGukqya+PxMbi/rmPYP4hn53pZ5udhoOMbV2kMMVzCWhg/3jz3DiGFkQCJLrp318O8ZD0YBX5jy006O</SesInfo>
<TokInfo>ginix1KzeO2bptUatvN+VEny6qjj53k7</TokInfo>
</response>

Take SessInfo and TokInfo and do:

curl -X GET http://192.168.8.1/api/security/dmz -H "Cookie: SessInfo-value"

You most likely will see something like this:
<?xml version="1.0" encoding="UTF-8"?>
<response>
        <DmzStatus>0</DmzStatus>
        <DmzIPAddress>192.168.8.100</DmzIPAddress>
</response>

So we need to alter DmzStatus and set it to 1:

curl -X POST -d "<request><DmzStatus>1</DmzStatus><DmzIPAddress>192.168.8.100</DmzIPAddress></request>" http://192.168.8.1/api/security/dmz -H "Cookie: SessInfo-value" -H "__RequestVerificationToken: TokInfo-value"

PS. Mind the space after "Cookie:".