Connecting NETGEAR MIMO Antenna ruined by 4G modem

I was getting good speeds on my NETGEAR 4g modem LB1120, but wasn’t getting full bars so I tried an antenna as instructions suggested, plugged in a MIMO NETGEAR connecter and it ruined everything somehow. When I saw it brought down my 50mbps speeds on my 4g modem to 1 mb, I disconnected it after trying various areas seeing it didn’t improve it. Oddly enough after I did this, all the devices on my plan where all brought down to 3g speeds. I have no idea how this antenna could have affected my phones but this happened immediately after, somehow the antenna actually ruined the connectivity in my house. Resetting devices brought back my 4g speeds on all my devices except the 4g modem. I’ve tried factory resetting it, unplugging, taking out SIM and putting it back, but I am stuck with 3g speeds when I did have 4g lte before the antenna. Rest assure I won’t ever use that NETGEAR antenna again but I really want to fix my modem that was great before all of this. The only idea I have left is replacing the modem with a new one.