LB2120 to cover alarm system only not entire network

I have Virgin hub 3 in modem mode, connected by RJ45 to this a TP link deco x60 for wifi distribution then a splitter off of that leading to another 2 decos (hardwired) and a alarm system. I only want this to cover the alarm system on backup. I don’t want to cover the entire network. Can I install this LB2120 after the splitter (inbetween the network splitter and alarm system)? Or does it have to go inbetween the Virgin hub and deco x60? Havnt bought yet. Just trying to determine if it meets my needs prior. Thanks