Here is what I did so far to get the router working
Uninstall personal firewall stuff and virus scanner, it's recommended to do this inside a virtual machine
Select correct files for RB760igs from OpenWrt page
Open device and solder serial terminal (e.g. use CH340 to USB module), this will save you a lot of time but you will lose warranty!
Press "x" on terminal to enter bootloader menu
enter p-1 = bootp
enter o-e = boot from Ethernet (must be changed later!)
enter b-2 = force backup-bootloader loading
enter e-yes = format storage (no fear, can be undone with Mikrotiks netinstall tool)
If you have problems with the network
try another cable
try another USB-Ethernet card
Set your network card to
IPv4 192.168.1.2
Subnet 255.255.255.0
gateway 192.168.1.1
Use TinyPXE
copy image "...initramfs..." into TinyPXE sub folder .../file/boot
select "HTTPd"
set Option 54 to 192.168.1.2
set IP Pool start to 192.168.1.3
set Next-Server to 192.168.1.2
set Option 1 to 255.255.255.0
set Option 3 to 192.168.1.1
set Option 6 to 192.168.1.1
set Option 28 to 192.168.1.255
enter the "...initramfs..." file into Filename text field
deselect "Filename if user-class=gPXE or iPXE"
select [Online]
Now let the router boot...
Router should boot with following message (or similar one)
RouterBOOT backup booter 6.46.3
RB760iGS
CPU frequency: 880 MHz
Memory size: 256 MiB
Storage size: 16 MiB
Press any key within 2 seconds to enter setup..
loading kernel... kernel not found
trying bootp protocol.... OK
Got IP address: 192.168.1.13
resolved mac address 00:50:B6:0A:B3:A8
transfer started ................................... transfer ok, time=4.44s
setting up elf image... OK
jumping to kernel code
[ 0.000000] Linux version 5.4.81 (builder@buildhost) (gcc version 8.4.0 (OpenWrt GCC 8.4.0 r15151-13961da6ce)) #0 SMP Sun Dec 6 08:18:07 2020
...
Copy "...sysupgrade..." file to router via scp
scp -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null openwrt-ramips-mt7621-mikrotik_routerboard-760igs-squashfs-sysupgrade.bin root@192.168.1.1:/tmp
Write image into NAND flash now
sysupgrade -v /tmp/openwrt-ramips-mt7621-mikrotik_routerboard-760igs-squashfs-sysupgrade.bin
Connect the router's WAN port to your common router and reset it to get internet access
opkg update
opkg install luci-ssl
opkg install luci-i18n-base-<language>
Hope this helps anybody, otherwise please ignore it!