Advice on tenant with thermostat for whole building

[Kaybreh Mathis]

Hello! My husband and I just bought a property. It was a large single family home and at some point 40 or so years ago was split up into 6 units. One unit houses the thermostat for the furnace which heats the whole building. The tenant living there frequently turns it way down and then all the other tenants complain because it drops to 55 degrees in their units. The tenant has repeatedly been told not to touch the thermostat. The tenant can’t be removed right now. So I have several questions. 1- is there some way I could control the thermostat without having to have wifi at the property? Like some kind of system that works off cell towers to allow me to set it from my phone? 2- how can I enforce this without evicting? Send a certified letter so they know I mean business? It’s really getting cold to constantly deal with complaints from other tenants, and then to spend hours trying to get ahold of the tenant responsible. How would you proceed? Thanks!