Alarm not fit for purpose

Just bought ring burglar alarm - advised to post here by support. It is basically useless without cellular plan which is additional cost? Am based in UK and bought alarm with app communication so can turn off and on remotely. Firstly no external siren but I can add one of these The main problem is my router is by the door. So if anyone breaks in they can disable the alarm by disconnecting my router. I expected that at least the alarm would still function ie a code is not entered and the siren (internal and external) would activate. According to support this is not the case. I find it hard to believe but that is what support tell me. No cellular plan leaves the alarm disabled if the wifi is disconnected before it fires off. About to take alarm back as on this basis is not fit for purpose. Or have I been given bad information by two advisors…. Very disappointing