There was a typo in your command (dtb instead of dbt). It should have been:
ln -sf /boot/armada-385-turris-omnia-sfp.dtb /boot/dtb
However even that doesn’t solve the problem by itself. After a reboot with the correct dtb I was greeted by this:
root@omnia:~# ethtool eth2
Settings for eth2:
Supported ports: [ MII ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Supported pause frame use: Symmetric
Supports auto-negotiation: Yes
Supported FEC modes: Not reported
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Advertised pause frame use: Symmetric
Advertised auto-negotiation: Yes
Advertised FEC modes: Not reported
Speed: 1000Mb/s
Duplex: Full
Port: MII
PHYAD: 0
Transceiver: internal
Auto-negotiation: on
Supports Wake-on: d
Wake-on: d
Link detected: no
root@omnia:~# dmesg | fgrep -i eth2
[ 4.467538] mvneta f1034000.ethernet eth2: Using hardware mac address d8:58:d7:00:3e:6e
[ 19.836832] mvneta f1034000.ethernet eth2: configuring for inband/sgmii link mode
[ 51.723326] mvneta f1034000.ethernet eth2: PHY [i2c:sfp:16] driver [mv88x3310]
root@omnia:~# dmesg | fgrep -i sfp
[ 13.909149] sfp sfp: module Turris RTSFP-10G rev A sn 2109150017 dc 15-09-21
[ 13.918516] sfp sfp: unknown connector, encoding 64b66b, nominal bitrate 10.3Gbps +0% -0%
[ 13.926914] sfp sfp: 1000BaseSX- 1000BaseLX- 1000BaseCX- 1000BaseT- 100BaseTLX- 1000BaseFX- BaseBX10- BasePX-
[ 13.937046] sfp sfp: 10GBaseSR+ 10GBaseLR- 10GBaseLRM- 10GBaseER-
[ 13.943350] sfp sfp: Copper length: unsupported/unspecified
[ 13.949121] sfp sfp: Options: txdisable, txfault, los+
[ 13.954462] sfp sfp: Diagnostics: ddm, intcal, rxpwravg
[ 51.712236] mv88x3310 i2c:sfp:16: PHY supports (18000,000070ea) more modes than phylib supports, some modes not supported.
[ 51.723326] mvneta f1034000.ethernet eth2: PHY [i2c:sfp:16] driver [mv88x3310]
root@omnia:~# ethtool -s eth2 port tp
root@omnia:~# ethtool eth2
Settings for eth2:
Supported ports: [ TP ]
Supported link modes: 10baseT/Full
100baseT/Full
1000baseT/Full
Supported pause frame use: Symmetric
Supports auto-negotiation: Yes
Supported FEC modes: Not reported
Advertised link modes: 10baseT/Full
100baseT/Full
1000baseT/Full
Advertised pause frame use: Symmetric
Advertised auto-negotiation: Yes
Advertised FEC modes: Not reported
Speed: Unknown!
Duplex: Unknown! (255)
Port: MII
PHYAD: 22
Transceiver: internal
Auto-negotiation: on
Supports Wake-on: d
Wake-on: d
Link detected: yes
root@omnia:~# ls -l /boot
-rw-r--r-- 1 root root 18834 May 25 21:00 armada-385-turris-omnia-phy.dtb
-rw-r--r-- 1 root root 18870 May 25 21:00 armada-385-turris-omnia-sfp.dtb
-rw-r--r-- 1 root root 1199 Dec 10 2021 boot.scr
lrwxrwxrwx 1 root root 37 Jul 10 20:29 dtb -> /boot/armada-385-turris-omnia-sfp.dtb
-rwxr-xr-x 1 root root 3379416 May 25 21:00 zImage
root@omnia:~#