Amateur radio records are 2651 Km at 1.3 Ghz and 1560 Km at 2.4 Ghz. Unusual but very low power can go a long way under “tropo” conditions.
On a recent flight in Switzerland I had constant 4g cover up to 7000 ft. Surprised me but was helpful to get weather updates
I flew to Switzerland for a trip to Friedrichshafen last month and in the cruise at FL090 I was able to get a 4G signal sporadically over France, sufficient to get weather and send an iMessage but it was more consistent from the Jura onwards.
In the mountains, 3G/4G works great. Overflying the Alps at say FL160, terrain some 6000ft below, I can be browsing the forum 
Funny …
On the way to La Rochelle at 3500 feet, I could not get a single bit of data. In the way back, I stayed at 2500 feet on a more easterly route and I got good 4G from the Loire valley to Paris.
Maybe a Cessna is a better Faraday cage than a TB or other low wing …
