Hello – my father died unexpectedly. One of the first things my mom asked me to do was replace a broken exterior door. I realized the alarm sensor for that door wasn’t even working and now realize there’s a lot about their security system that needs updating – long story short the system was installed 20 years ago and updated via ADT. At some point a few years back my father added a keychain activate/deactivate remote and switched monitoring companies to a third-party, I believe the third-party is overcharging for their monitoring service. I’d also like to add LTE/5g to their system as I believe it is currently only over phone line (how can I check?) My mother would also like some way to check the alarm status remotely via an app and I also need to add back the missing door sensor. Where do I begin? Their system is a PC 1864, they also have an ADT-branded RFK5500-433 panel, and about 20 door/window sensors as well as a wired motion sensor. I believe their current contract ends in december, but I get the sense the monitoring company will make switching away from them difficult. What is the cheapest and best way forward to update the system as above and also get a new affordable monitoring company? Thank you.