Look, it has to do with how AV-Test and AV-Comparatives work. While I trust them for PC antiviruses, I don't care a whit about their Android findings. First and foremost, the way they test is they keep a library of known "malware", some of which aren't really malware by the way, and see which app detects the most of them. Now some of these are considered malware because of the potential they have. Note your warning that it's labeled as "RISKWARE", which means it may be legit, it may be not.
This service (SMSReg) in particular works as a way for the app to send a message without interaction from the user. This is riskware because on some Chinese phones, this has been placed on the firmware for ads and stuff, using the person's credits to send messages to get ads, giving earnings to the manufacturer at the cost of the user's money. However, there is also a legit use for this service. Some apps have this for security. Say your phone gets stolen, and you have the banking app in there. Once the app receives an order via a certain condition (received a message with an instruction code from you, or the sim card of the phone was replaced, etc), this allows the app to send you an SMS with certain info, like IMEI of the phone, the phone's location, etc. If this service was not available, then the banking app cannot send SMS without interaction from the thief. This allows for behind the scenes SMS reporting without the thief knowing that his location is being reported.
So I'll suggest at checking the features of the app, if it has the capability to send SMS on it's own, check your credits if you are losing them without you knowing, and contact the devs.