The boot protocol is quite different between TurrisOS and OpenWRT.
If you have OpenWRT 19.07 installed, sysupgrade should have set this u-boot environment variable for you:
# fw_printenv openwrt_mmcload
openwrt_mmcload=setenv bootargs "$openwrt_bootargs cfg80211.freg=$regdomain"; fatload mmc 0 0x01000000 zImage; fatload mmc 0 0x02000000 armada-385-turris-omnia.dtb
So, by default u-boot tries to load armada-385-turris-omnia.dtb from the FAT filesystem in /dev/mmcblk0p1, which is the boot partition and not mounted by default. Putting something in /boot will not have any effect.
I suggest you mount /dev/mmcblk0p1 somewhere, and copy TurrisOS armada-385-turris-omnia-sfp.dtb over there, next to zImage and armada-385-turris-omnia.dtb. And then,
# fw_setenv openwrt_mmcload 'setenv bootargs "$openwrt_bootargs cfg80211.freg=$regdomain"; fatload mmc 0 0x01000000 zImage; fatload mmc 0 0x02000000 armada-385-turris-omnia-sfp.dtb'
Next time, u-boot should load the device tree for SFP.
(The experimental SFP support mentioned in the thread linked above does essentially the same thing, but you would have to build your own image)