I've finally got down to the root cause of my connection problem with secondary sim is:
That it's actually secondary modem that seems to be the root cause for 1 month of issues
1. first i tried to swap vendor (after having intermittent issues (ping fails - throwing line down/up all the time) even with a really good signal and speedtest)
2. then got external antennas (same issue)
3. playing with bands and manual settings (not helping)
4. replacing sim for secondary modem (replacing carrier with another (3rd) carrier). NOW I only get 3G with secondary modem, I swap SIM's between primary and seconday modem, 3G stays at secondary modem (but with another carrier), so I get 3G on secondary modem regardless of SIM/carrier.
5. I swap external antennas from modem1 to modem2 and vice versa (thinking one antenna must be broken) - problem still stays with secondary modem. - I align antennas direction to take that out of the equation.
6. I swap back SIM (proving antenna/direction/sim combo - is not the issue here) and problem still stays with secondary modem
7. I try antennas included with router. still same problem stays with secondary modem.
8. I move back to the 1st SIM (the one with the best signal in this area where iam current at - now I have get 4G LTE with secondary modem again) but I also have the issue with failing pings rendering loadbalancing / failover useless.
Tried reboot modems and router multiple times between all the steps ...
Ideas? - can it be secondary modem has some kind of internal antenna defect ?