Well, it really depends.
First thing is what are really your constraints. You said the operator doesn't allow you to use CoAP/M2M so you have to stick with raw UDP but there are many other protocol stacks supported by the Quectel BC95, according to this official datasheet, it can use all of these:
So you can ...
Direction detection is one of the selling points for UWB (Ultra Wide Band) and Bluetooth 5.0 when combined with multiple antenna arrays.
This allows for AoA (Angle Of Arrival) to be calculated which will give you a relative direction to another device.
But given the form factor of the a phone it only really allows for the antenna separation in a single plan ...
This problem can't easily be solved without being able to install software on both devices.
Devices usually have screen on front, and camera on front and back. So if you were to use screen to show a QR code, and both cameras on other device to look for that QR code, you'd be able to determinate relative position (based on angle and size of QR code seen on ...
These systems use multiple antenna, not multiple distinct radios to compute angle of arrival based on time of arrival at each antenna.
It's probably worth noting that these radios tend to be SDR (software defined radios) which detect signals by doing frequency analysis on a whole chuck of spectrum, rather than running to a specific frequency. This makes ToA ...