How to connect CPU with RGMII pins to an LTE module that takes only SGMII signals?
Serial requires transceivers and restructuring of the serial data to slower parallel data streams, which means you need logic.
An Marvell Alaska 88E1512 might work, it claims you can go from SGMII to RGMII.
EC21 (SGMII pins)&lt;--&gt; 88E1512 &lt;--&gt; Hi3559A (RGMII pins)
Which is a lot simpler than using two phys to translate.

Source: https://www.marvell.com/transceivers/assets/Alaska_88E1512-001_product_brief.pdf