Hi,
I check my signal strengh in my worst placed shellys.
Bilen WiFi RSSI: -73 dBm Worst, sitting in my garage one floor down from the access point
Entre WiFi RSSI: -66 dBm Quite bad one floordown from the access point.
-73 dBm is much worse than -48dBm, and mine are working fine.
However this is only the part of the answer if the wifi should work fine.
(I’m no Wifi expert, but picked up something about radio during my 30 years in Mobile business.)
Firstly this (-48dBm) is the signal strenght that the Shelly receives, there has to be balance between sending and receiving, most likely, the shelly has less out put power and worse antennas than your access point. This could mean that your access point might reach your shelly, but the shelly might not have the power to reach to access point.
More likely, you have interference, Wifi rfequences are a shared resource, even if I live in a house with quite some distance they some times interfere with my Wifi. More than that, My son some times place forthnight, watch youtube at the same time as he skype with his friends. This fills up the radio waves. This is partly why I changed access points to dual band, moving all the heavy videos to 5Ghz, leaving shellys and others on 2.4Ghz. This would also explain why you only have problems some times, (maybe the neighbours gaming and video night with her friends). If you want to see how wifi is used in your neighbour hood you can down load a “wifi analyzer” on you mobile and how "clean the frequencies you are using are.
Still regardless, I don’t think the shelly should hang because of this, maybe appear a bit slow or in worst case not accessable for some time.
Still believe more in the firmware, or even your hardware.
Best Regards
Tobe