According to WinFuture (with auto translation from the native German):
As the head of Microsoft's hardware team, Panos Panay said yesterday at an event in Berlin, one plans to equip the Surface Duo and the Surface Neo with support for the new mobile radio standards from the 5G range. At the same time, he indicated that it will probably take a while. A 'good' camera will be in the devices in the first generation, assured the Microsoft manager.
As Panay explained on questions from a journalist during a panel discussion, there is an ambitious roadmap for the introduction of 5G-capable devices, but at the moment he does not want to comment on the details. However, it is certain that both the Surface Duo with its two 5.6-inch displays, and the Surface Neo with the two 8-inch panels, will hit the market with support for 4G/LTE, at least.According to Panay, the dual-screen smartphone and the dual-screen tablet will support LTE-Advanced, with the two new products also being "future 5G devices". Exact information will also be provided on the timetable for introducing such variants "when the time is ripe"...
...Given the very flat design of the two body halves of the Surface Duo and the energy needs of Qualcomm's external 5G modem for the Snapdragon 855 used, using such a combination would not be very effective given the larger size of battery needed.
It would therefore be conceivable that Microsoft first waits for the availability of the next generation of Qualcomm's high-end or at least mid-range SoCs, which will then come along for the first time with a directly integrated 5G modem.
Qualcomm's next mobile chipset will probably be the Snapdragon "865" (SM8250), which will come with a 5G variant, or the Surface Duo might use the Snapdragon 735 (SM7250), WinFuture reckons, which will also be 5G-enabled.
In summary, we'll have the Surface Duo with an 855 and LTE for Q4, 2020, and then a Duo 5G with a 865 nd 5G for Q4 2021. It all seems a long way off to me, though!