[LG V20 ROMs, Kernels, Recoveries, & Other Developm] el-sahef:
I tried it with method 2 on H990N20h_00_OPEN_HK_DS_OP_0225.kdz and can report success.

But I could not get into the system with recovery-twrp-h990-2019-10-25.img, it gave me this secure startup screen were you can try a password 30 times. Also tried older versions, but the one that worked in the end was twrp-3.0.2-1-h990.img which is also used during the dirty-santa process.

I suppose I could flash a newer one now after I did the initial android setup process but I did not try it yet.

For safety, I read back the written aboot and recovery partitions and verified them against the files before powering off the phone. Also I filled up twrp-3.0.2-1-h990.img with zeros up to 42.467.328 Bytes file size (same as partition size) for easy verification.

In the root shell:

elsa:/ # dd if=/storage/emulated/0/Download/aboot_new.img of=/dev/block/bootdevice/by-name/abootbak
/emulated/0/Download/aboot_new.img of=/dev/block/bootdevice/by-name/abootbak <
4096+0 records in
4096+0 records out
2097152 bytes transferred in 0.182 secs (11522813 bytes/sec)
elsa:/ # dd if=/storage/emulated/0/Download/aboot_new.img of=/dev/block/bootdevice/by-name/aboot
mulated/0/Download/aboot_new.img of=/dev/block/bootdevice/by-name/aboot <
4096+0 records in
4096+0 records out
2097152 bytes transferred in 0.167 secs (12557796 bytes/sec)
elsa:/ # dd if=/storage/emulated/0/Download/twrp-3.0.2-1-h990ext.img of=/dev/block/bootdevice/by-name/recoverybak
.0.2-1-h990ext.img of=/dev/block/bootdevice/by-name/recoverybak <
82944+0 records in
82944+0 records out
42467328 bytes transferred in 3.341 secs (12710963 bytes/sec)
elsa:/ # dd if=/storage/emulated/0/Download/twrp-3.0.2-1-h990ext.img of=/dev/block/bootdevice/by-name/recovery
.0.2-1-h990ext.img of=/dev/block/bootdevice/by-name/recovery <
82944+0 records in
82944+0 records out
42467328 bytes transferred in 3.286 secs (12923715 bytes/sec)
elsa:/ # dd if=/dev/block/bootdevice/by-name/recovery of=/storage/emulated/0/Download/recovery_new_verify.img
ery of=/storage/emulated/0/Download/recovery_new_verify.img <
82944+0 records in
82944+0 records out
42467328 bytes transferred in 1.053 secs (40329846 bytes/sec)
elsa:/ # dd if=/dev/block/bootdevice/by-name/recoverybak of=/storage/emulated/0/Download/recoverybak_new_verify.img
erybak of=/storage/emulated/0/Download/recoverybak_new_verify.img <
82944+0 records in
82944+0 records out
42467328 bytes transferred in 1.043 secs (40716517 bytes/sec)
elsa:/ # dd if=/dev/block/bootdevice/by-name/abootbak of=/storage/emulated/0/Download/abootbak_new_verify.img
bak of=/storage/emulated/0/Download/abootbak_new_verify.img <
4096+0 records in
4096+0 records out
2097152 bytes transferred in 0.056 secs (37449142 bytes/sec)
elsa:/ # dd if=/dev/block/bootdevice/by-name/aboot of=/storage/emulated/0/Download/aboot_new_verify.img
of=/storage/emulated/0/Download/aboot_new_verify.img <
4096+0 records in
4096+0 records out
2097152 bytes transferred in 0.056 secs (37449142 bytes/sec)
elsa:/ #

The verify-files showed up in the file manager on the phone but not on the computer to which the phone was connected. I had to move them to another folder (e. g. DCIM) with the file manager on the phone first to be able to access them with the computer.

After successful verification, I powered off the phone, switched it on again with the button combination to get into twrp and then did everything as usual.

Unfortunately it did not solve the problem with the second SIM card not working on my H990N after rooting. Is there anyone who has a rooted H990N working with both SIM cards?

I bought it to have a spare for my H990DS which works fine with root and both SIM cards but if I can not sort this out I will probably have to sell it again and get a second H990DS instead because I need a rooted phone and dual SIM.