Cellular reception of my mobile phone company's network has always been weak (on 3G; forget about 4G) in the house we moved into 2y ago but the past 48 hours I'm barely getting beyond the "No Signal" ...

Turning your phone's connection off and then back on is the quickest and easiest way to try and fix your signal woes. If you're moving around from one location to another, toggling Airplane mode ...