By chance my Picaxe mate Andrew lives near Mt. Taranaki & may be tempted to take a stroll up there for LoRa™ trials.
Is there such a thing as LOS envy ?
I don’t see 280km as a particular problem, but you would want to (at the chosen data rate) work out how many dBm is needed to complete the link.
This is not difficult, just have the transmitter send packets starting at 17dBm and drop 1dBm at a time all the way down to 2dBm. Have the receiver beep once for a packet RX and again for a CRC fail.
At the start of the sequence (17dBm) send out an FM tone at full power which you will hear on a UHF handheld then just count the single beeps, you could even log the RSSI on a PC.
LOS for a foil party baloon at 7000M is about 340km. As the humble RFM22B was good on my PICO balloon tracker up to 174km (using an LNA and gain vertical) 340km ought to be easy enough with a LoRa device.
The bigger latex balloon go up to around 30000M, LOS there would be around 700km.