Separate modem isn't a bad idea actually, could keep the current router then until replacing it at a later date. Very brief looking I can see some that have 4 antennas from either MicroTik or Teltonika. Current 4G router has 2 antennas and I have got some pretty large antennas to improve the signal, if you have 4 can you mix different antennas or better that they are all the same?

Might be costly though, they appear to be for more business use.