Looking into it.
First, you can do curl 'https://g2master-sa-east.tclclouds.com/check.php?mode=2&cktp=2&cltp=10&type=Firmware&id=[IMEI]&curef=[CU]&fv=[VER]'
Where:
IMEI is your IMEI
CU is DISTRIB_FOTA_CU variable from /etc/openwrt_release in the OEM firmware (HH500V-2XVDESX in my case)
VER is DISTRIB_FOTA variable from /etc/openwrt_release in the OEM firmware. If you try with lower numbers it will tell you possible firmware upgrades. In my case, it worked with 020010.
Getting the firmware is a little bit more complex:
args = "id={imei}&salt={time in milliseconds in 13 digits}123456&curef={cu}&fv={currentversion}&tv={finalversion}&type=Firmware&fw_id={FW_ID tag from XML}&mode=2&cltp=10"
string = args + "1271941121281905392291845155542171963889169361242115412511417616616958244916823523421516924614377131161951402261451161002051042011757216713912611682532031591181861081836612643016596231212872211620511861302106446924625728571011411121471811641125920123641181975581511602312222261817375462445966911723844130106116313122624220514"
args += "&vk=" + sha1(string).hex().upperCase()
curl -d "${args}" "https://g2master-eu-west.tclclouds.com/download_request.php"
It will return some XML data with a list of host and the URL for those hosts.