I remotely monitor a mountain house in a snowy area, and this is what I use:
-Monitored security, fire and CO alarm with web interface to arm/disarm remotely
-Whole house water leak detector with automatic shutoff (FloLogic)
-Web enabled video cameras pointed at driveway, inside garage, boiler control panel (to confirm it's working), exterior roof (to monitor snow load) (Logi Circle)
-Web enabled thermometers inside and outside house with push notifications (Eve and First Alert)
-Web enabled door locks with multiple codes, remote locking/unlocking and push notifications (Kwikset)
-Web enabled garage door openers for remote opening/closing with push notifications (Chamberlain)
-Web enabled and programmable light switches (iDevices)
-Web enabled outlets for critical components so I can reboot them if they go offline (iDevices)
-Web Enabled landscaping sprinkler controller (Rain Bird)
-A power outlet for the modem and router that automatically cycles off and on every day (NetReset)
I can lock and unlock doors for service providers remotely if it's just a one-time service. If the power goes out, the alarm and leak detector have battery backups but everything else will go offline. The other single point of failure is the modem/router, which is why I have it set up to reboot every night (the outlet uses an internal timer and is not dependent on remote access). If everything is offline for an extended period of time, my failsafe is that I have pre-programmed codes into the door locks that I can give to service providers to enter just the garage or the garage and house. I give door lock codes to vendors, but not the alarm code. Instead, I disarm the alarm myself remotely prior to the service visits.
Think ahead about compatibility between devices before you start buying them. I have standardized on Apple HomeKit devices, but there are other options and every manufacturer also has their own monitoring app.