It's not clear in your opening post, when you say the internet "goes to muck", are you talking about lack of signal or lack of bandwidth.
Also not sure, why you want another router, different provider ?
Anyway
If you have good signal i.e "bars" on your phone or hotspot, but crap broadband speeds, then an external antenna is of limited use in the first place.
The major limiting factor for bandwidth assuming you have a signal is contention, i.e how many users at the same time.
i.e on a Sunday evening, when everybody is online, speed sucks, 4:00 am, all in bed, same equipment, great speeds.
The purpose of the antenna is to boost your signal strength, i.e if you have poor signal indoors at your modem, you use an external antenna to gather as much signal from the great outdoors and get it into your modem.
In this case a second antenna is required if possible, as in the great scheme of things, the antenna should not be all that expensive. Trying to use the same antenna involves "splitting" the cable obviously, and regardless of how you do this there will be signal loss.
So in layman's terms at your end terminal , ie laptop, ip camera, phone etc there are 3 variables.
1. The actual 4G/LTE signal from your mobile provider, this is the bit the external antenna mostly helps with if you have a poor signal level.
2. The contention ratio of your provider, i.e how many users on the mast, nothing you can do about this only move provider if you had a choice.
3. The quality of the distribution of the wireless signal in your home, i.e wifi strength and speed, this can be helped/hindered by the quality of your network equipment