So I finally got my hands on a serial cable that works with my Mikrotik.
I tried to format NAND through the BIOS as you suggested.
After that tried booting 19.07.RC2 but I couldn't get it to boot.
Booted 18.06. and then upgraded to 19.07.
After that the device keeps going in loop.
In the console I can see this:
[ 3.054043] NAND flash driver for RouterBoard 4xx series version 0.2.0
[ 3.060874] nand: device found, Manufacturer ID: 0xad, Chip ID: 0xdc
[ 3.067234] nand: Hynix NAND 512MiB 3,3V 8-bit
[ 3.071666] nand: 512 MiB, MLC, erase size: 256 KiB, page size: 2048, OOB size: 64
[ 3.079310] Scanning device for bad blocks
[ 3.158919] Bad eraseblock 479 at 0x0000077ff800
[ 3.375876] Bad eraseblock 1844 at 0x00001cd3f800
[ 3.403720] Bad eraseblock 1993 at 0x00001f27f800
[ 3.417289] Creating 3 MTD partitions on "NAND 512MiB 3,3V 8-bit":
[ 3.423477] 0x000000000000-0x000000040000 : "booter"
[ 3.429995] 0x000000040000-0x000000400000 : "kernel"
[ 3.436296] 0x000000400000-0x000020000000 : "ubi"
[ 3.446603] libphy: Fixed MDIO Bus: probed
[ 3.453884] IP17xx: Found IP175D at ag71xx-mdio.0:00
[ 3.461449] libphy: ag71xx_mdio: probed
[ 3.806071] ag71xx ag71xx.1: connected to PHY at ag71xx-mdio.0:04 [uid=02430d80, driver=Generic PHY]
[ 3.815743] eth0: Atheros AG71xx at 0xba000000, irq 5, mode: rmii
[ 4.259683] ag71xx ag71xx.0: connected to PHY at ag71xx-mdio.0:00 [uid=02430d80, driver=IC+ IP17xx]
[ 4.269293] eth1: Atheros AG71xx at 0xb9000000, irq 4, mode: mii
[ 4.277204] NET: Registered protocol family 10
[ 4.285926] Segment Routing with IPv6
[ 4.289669] NET: Registered protocol family 17
[ 4.294140] bridge: filtering via arp/ip/ip6tables is no longer available by default. Update your scripts to load br_netfilter if you need this.
[ 4.307126] 8021q: 802.1Q VLAN Support v1.8
[ 4.311423] rb: no calibration data found
[ 4.318830] UBI: auto-attach mtd6
[ 4.322148] ubi: refuse attaching mtd6 - MLC NAND is not supported
[ 4.328369] UBI error: cannot attach mtd6
[ 4.333000] VFS: Cannot open root device "(null)" or unknown-block(0,0): error -6
[ 4.340534] Please append a correct "root=" boot option; here are the available partitions:
[ 4.348877] 1f00 44 mtdblock0
[ 4.348881] (driver?)
[ 4.355418] 1f01 4 mtdblock1
[ 4.355421] (driver?)
[ 4.361943] 1f02 8 mtdblock2
[ 4.361946] (driver?)
[ 4.368473] 1f03 4 mtdblock3
[ 4.368476] (driver?)
[ 4.375002] 1f04 256 mtdblock4
[ 4.375005] (driver?)
[ 4.381530] 1f05 3840 mtdblock5
[ 4.381532] (driver?)
[ 4.388059] 1f06 520192 mtdblock6
[ 4.388062] (driver?)
[ 4.394579] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
[ 4.403707] Rebooting in 1 seconds..
This part here seems strange: "ubi: refuse attaching mtd6 - MLC NAND is not supported"
Do you know what this means?