  The recent launch of Einstein Probe (EP) in early 2024 opened up a new window
onto the transient X-ray sky, allowing for real-time discovery and follow-up of
fast X-ray transients (FXRTs). Multi-wavelength observations of FXRTs and their
counterparts are key to characterize the properties of their outflows and,
ultimately, identify their progenitors. Here, we report our long-term radio
monitoring of EP240315A, a long-lasting ($\sim 1000$ s) high redshift ($z=4.9$)
FXRT associated to GRB~240315C. Our campaign, carried out with the Australian
Telescope Compact Array (ATCA), followed the transient's evolution at two
different frequencies (5.5 GHz and 9~GHz) for three months. In the radio
lightcurves we identify an unusual steep rise at 9 GHz, possibly due to a
refreshed reverse shock, and a late-time rapid decay of the radio flux, which
we interpret as a jet break due to the outflow collimation. We find that the
multi-wavelength counterpart of EP240315A is well described by a model of
relativistic jet seen close to its axis, with jet half-opening angle $\theta_j
\approx 3 ^{\circ}$ and beaming-corrected total energy $E \simeq 4\times
10^{51}$~erg, typical of GRBs. These results show that a substantial fraction
of FXRTs may be associated to standard GRBs and that sensitive X-ray monitors,
such as Einstein Probe and the proposed HiZ-GUNDAM and Theseus missions, can
successfully pinpoint their relativistic outflows up to high-redshifts.
