Can mobile reception 4G/5G signal reception be affected by atmospheric changes?
Yes, microwave propagation, like any other electromagnetic wave, is affected by medium properties – especially moisture/water content.

As a tendency, the higher you go in frequency, the more you notice the atmospheric absorption, but: there's specific frequencies that water absorbs especially well - these happen to be ~2.45 GHz and ~60 GHz: exactly the frequencies of Wifi. That has a causal relationship: These frequencies are less reliably useful due to exactly this absorption, so regulators agreed to having them usable by unlicensed transmitters (wifi, bluetooth, doorbells...). Microwave ovens work at 2.45 GHz – exactly because water absorbs energy at that frequency so well.

Now, you're asking about 4G and 5G: these use frequencies that were deliberately chosen not to be too strongly absorbed by atmosphere – after all, network operators pay billions for their licenses! So, yes, these effects do exist – and when you happen to end up in a dense snowstorm, you'll see how gravely.