Well, I don't think that Libreelec is a root of your trouble... Instead, it sounds like your USB multi-HDD box (in fact it's probably not a NAS, which means Network Attached Storage) maybe generates some high-frequency noise which may interfere with your WiFi network. I don't know how your device looks like exactly, it might be the external Power Supply of that box... It should have a metal shielding inside, under the plastic cover, but there are some cheap products that don't respect any construction rules... Don't know if it's the case but maybe yes... And it's possible it may change over the time due to parts degradation, especially capacitors... Also a different WiFi devices may have a different sensitivity to a noise or a too low power to overcome it..
Try to move the devices a bit or try to make a temporary shield to your USB box / box power supply using aluminium foil (carefully, I do NOT mean to open the boxes!) and check tf there's any difference.