LB1120 Is there a trick to modem recognizing external antenna?

Hi,

I've happily been using this LTE modem for 4 months.  Other than the signal strength of my cell provider, it's been very stable.  Recently I decided to install a MIMO antenna on the roof and hook it up to the modem's external antenna ports.  But I'm seeing some weird behavior that lead me to believe the modem doesn't always recognize there's an external antenna hooked up and uses its internal one instead!  Let me explain:

Before connecting the external antenna, I was usually getting 1-2 bars and my "RSRP" was around -115 - i.e. not very good reception.  After I connected the external antenna, my modem showed 4 bars and an RSRP of -99.  Much better.  But since then I've had to disconnect/reconnect the modem a couple times (not related to reception) and after I turned the modem back on and reconnected the external antenna, I'd see 2 bars w. -115 RSRP, making it look like the modem was still using the internal antennas rather than the connected external one.  The first time this happened, I did some combination of pushing the power button, disconnecting the actual power wire and disconnecting/reconnecting the external antenna and it went back to 4 bars/-99 RSRP, but I don't know what did the trick.   The second time this happened, I have thus far not been able to get to 4 bars/-99 RSRP again :-(

How do I make sure the modem recognizes and uses the external antenna instead of the internal one?  Any suggestions are appreciated.