So, after almost a week of testing with the new hardware board that I made, I have not encountered any problems. In the new hardware board, the solder joints are cleaner and all flux residue has been thoroughly cleaned. The wiring is same as before. So, it can be concluded that it is definitely not a problem of the arduino, voltage regulator or RFm95W module.
The only possible cause of error that was causing the earlier board to fail seems to be a connection problem as @srnet had suggested from the very beginning. However, I'm unsure if this connection problem was being caused by the flux residue. After watching a couple of videos on the topic, I found out that flux residue can cause problems and unwanted behavior. But, is it supposed to happen after just a couple of weeks/days of use or does it happen after a time span of months? This is the only thing that is unclear to me.
Thanks @srnet and @TomGeorge for your support!