This 4G router sometimes loses internet connectivity, while my smartphone, using  a SIM card from the same internet provider seems to have much more reliable connection.

Just a while ago for example, the router had  no internet connection but my smartphone's SIM was working fine.

The lights on the router were  as follows: The globe internet icon was ON, but the 3 bar signal strength indicators were ON for about 3 seconds then OFF for a similar time, then continuing to turn on and off as described.

Now this is what is happening. The signal bars: 3 lights stay on for a few seconds, then change to one bar, then off, then repeat - and  now I just saw the change from 3- 2- 1- OFF, repeatimg like that. So not always in exactly the same order. On this router, I am using a MIMO external antenna, basically similar to the antennas I removed from the router, but bigger in size.

Then some time later connectivity returned to the router, but with the signal lights changing from 1 to 2 bars, to off for very short time, sometimes also going to 3 bars.

Of course, my smartphone only has its built-in antenna, but just now seems to give me a much more reliable connection than this particular TP Link router.

Any ideas? Or Is it possible that the router goes into a kind of sleep mode in periods of inactivity?