[LG V20 ROMs, Kernels, Recoveries, & Other Developm] el-sahef:
With dirty santa it is the same problem. As soon as the phone is rooted there is no network on the second sim card.

Also tried flashing a complete H990ds kdz and tons of other things, nothing helped.

At this point I suspect it can not be done on H990N with both SIM cards working and most people using this model never checked if the second SIM card slot was working with root because they did not use the second SIM functionality at all.

For some time now I had the suspicion that the engineering bootloader does something that prevents the second SIM card from working on the H990N. So I tried to verify this theory by flashing only the engineering bootloader and keep the stock kernel. In that case, I get the static screen problem so checking the cellular network status on the phone itself is impossible. But I found a way to do it via adb: https://stackoverflow.com/questions/28537468/getting-cellular-network-status-via-adb-shell

So I checked the value of "mServiceState" by using

adb shell dumpsys telephony.registry

and this is the relevant part of the output with the original bootloader:

last known state:
Phone Id=0
mCallState=0
mCallIncomingNumber=
mServiceState=1 1 voice home data home Unknown Unknown CSS not supported -1 -1 RoamInd=-1 DefRoamInd=-1 EmergOnly=false IsDataRoamingFromRegistration=false IsUsingCarrierAggregation=false m
LteEarfcnRsrpBoost=0 mIsVoiceSearching=false mIsDataSearching=false mVoiceNetworkName=null mDataNetworkName=null Check64QAM0 Dual carrier0
mVoiceActivationState= 0
mDataActivationState= 0
mSignalStrength=SignalStrength: 99 0 -120 -160 -120 -1 -1 99 2147483647 2147483647 2147483647 2147483647 0 2147483647 gsm|lte 0 0 0 2147483647 2147483647
mMessageWaiting=false
mCallForwarding=false
mDataActivity=0
mDataConnectionState=-1
mDataConnectionPossible=false
mDataConnectionReason=roamingOff
mDataConnectionLinkProperties=null
mDataConnectionNetworkCapabilities=null
mCellLocation=Bundle[{cid=-1, lac=-1, psc=-1}]
mCellInfo=null
Phone Id=1
mCallState=0
mCallIncomingNumber=
mServiceState=0 0 voice home data home netzclub+ netzclub+ 26203 netzclub+ netzclub+ 26203 LTE LTE CSS not supported -1 -1 RoamInd=-1 DefRoamInd=-1 EmergOnly=false IsDataRoamingFromRegistration=f
alse IsUsingCarrierAggregation=false mLteEarfcnRsrpBoost=0 mIsVoiceSearching=false mIsDataSearching=false mVoiceNetworkName=null mDataNetworkName=null Check64QAM0 Dual carrier0
mVoiceActivationState= 0
mDataActivationState= 0
mSignalStrength=SignalStrength: 99 0 -120 -160 -120 -1 -1 22 -93 -7 156 2147483647 0 2147483647 gsm|lte 14 0 0 2147483647 2147483647
mMessageWaiting=false
mCallForwarding=false
mDataActivity=0
mDataConnectionState=-1
mDataConnectionPossible=true
mDataConnectionReason=dataAttached
mDataConnectionLinkProperties=null
mDataConnectionNetworkCapabilities=null
mCellLocation=Bundle[mParcelledData.dataSize=64]
mCellInfo=null
mConnectedApns=[[], []]
mPreciseDataConnectionState=Data Connection state: 0, Network type: 13, APN type: dun, APN: null, Change reason: dataAttached, Link properties: null, Fail cause:
mPreciseCallState=Ringing call state: -1, Foreground call state: -1, Background call state: -1, Disconnect cause: -1, Precise disconnect cause: -1
mCarrierNetworkChangeState=false
mRingingCallState=0
mForegroundCallState=0
mBackgroundCallState=0
mVoLteServiceState=VoLteServiceState: 2147483647
local logs:
(...omitted...)

This is with only one SIM card installed in slot 2 which corresponds to Phone Id=1. The line mServiceState=0 shows that the phone is connected to the cellular network (the network name "netzclub+" is also shown).

And this is the output when the engineering bootloader is installed:

last known state:
Phone Id=0
mCallState=0
mCallIncomingNumber=
mServiceState=1 1 voice home data home null null null null null null Unknown Unknown CSS not supported -1 -1 RoamInd=-1 DefRoamInd=-1 EmergOnly=false IsDataRoamingFromRegistration=false IsUsingCa
rrierAggregation=false mLteEarfcnRsrpBoost=0 mIsVoiceSearching=false mIsDataSearching=false mVoiceNetworkName=null mDataNetworkName=null Check64QAM0 Dual carrier0
mVoiceActivationState= 0
mDataActivationState= 0
mSignalStrength=SignalStrength: 99 -1 -1 -1 -1 -1 -1 99 2147483647 2147483647 2147483647 2147483647 0 2147483647 gsm|lte 0 0 0 2147483647 2147483647
mMessageWaiting=false
mCallForwarding=false
mDataActivity=0
mDataConnectionState=-1
mDataConnectionPossible=false
mDataConnectionReason=roamingOff
mDataConnectionLinkProperties=null
mDataConnectionNetworkCapabilities=null
mCellLocation=Bundle[{cid=-1, lac=-1, psc=-1}]
mCellInfo=null
mConnectedApns=[[]]
mPreciseDataConnectionState=Data Connection state: 0, Network type: 0, APN type: dun, APN: null, Change reason: roamingOff, Link properties: null, Fail cause:
mPreciseCallState=Ringing call state: -1, Foreground call state: -1, Background call state: -1, Disconnect cause: -1, Precise disconnect cause: -1
mCarrierNetworkChangeState=false
mRingingCallState=0
mForegroundCallState=0
mBackgroundCallState=0
mVoLteServiceState=VoLteServiceState: 2147483647
local logs:
(...omitted...)

The block Phone Id=1 which corresponds to the second SIM slot is not even there anymore.

Because in this test nothing besides the bootloader was changed (locked bootloader to engineering bootloader from dirty santa) and everything else was left untouched I conclude that the engineering bootloader is the reason for the nonworking second SIM functionality on this H990N and therefore having root and second SIM working is impossible.

So I will get rid of the H990N and get a second H990ds instead.