The USB-C connection on the back of the KVD21 (Home Internet), can there be an external antenna attached?

Go to you tube and search on "Tmobile home internet external antenna. You will find a number of videos concerning this. It looks like you can do it, but you do have to take the unit apart in order to connect it.

Allan

An external antenna can be connected to the T-Mobile gateway be it the Nokia or the Arcadyan but only via the connectors inside the shell of the router. Nater Tater has some good YouTube videos and you can also find pretty detailed information from waveform.com. The T-Mobile gateways can have a 2X2 MIMO or a 4X4 MIMO antenna of various types connected. If you are receiving the n71 5G frequency the 2X@ MIMO is sufficient. If you are receiving the n41 5G frequency then a 4X4 MIMO antenna is needed to take full advantage of the n41. You do have to take extreme care when attaching the antenna connections as the connectors are small and delicate. If you break one in the process it can be replaced but without the tools and technical expertise not an easy task. Such a repair would cost probably $100. If you send a broken gateway back to T-Mobile they can charge you for the cost of the gateway so do be careful if you go there.

The USB-C connector is an interface on the Ethernet LAN. So it is nothing but an Ethernet port on the LAN like the wireless connections only with wires. 

Hi Allan.. Oh, I have watched that video several times but once you take it apart.. you own it. And with the cost of everything over the moon, I don't want to buy the device. But, is there such a device as a signal 'booster' that can bump up the incoming signal and focus it on the internal antennas?

There are cellular boosters but they are not designed for such a purpose.  If you want a 5G cellular booster it is not cheap. Cellular boosters for 4G LTE are in the neighborhood of $500-600 for a fairly solid unit. The company weBoost has some fairly well rated cellular booster systems. You need to confirm what the antenna can do with the specific cellular signal you are trying to amplify. You really need a dual antenna solution to get better reception. There are many options out there and some are much better than others. Costs vary widely.

Don't negate 3 bars signal strength.  That's all I can get, but with great down speeds!

The signal strength is only part of the equation. Look at the cellular metrics in the mobile application. Signal strength is good but you need to know if the RSRP signal quality and SINR noise are also in the good to excellent range. If signal quality is poor and there is noise the performance will blow. External antenna solutions can help improve the signal in many cases. Waveform.com is a good source for information and their technical people are really helpful. 

I wish T-Mobile would implement a built-in antenna so we don’t have to open the gateway.

I have seen other cellular routers that do have external antenna connections and a robust feature set, but they are more expensive and T-Mobile would probably not allow the connection to their network. The current "solution" is about control on various levels and limitations on support. It would be nice if they did offer a router with the external antenna connections for subscribers in challenged locations where service could be improved. 

The ability to have an external antenna would really help my situation

I can see the same here. For many users the use of an external antenna would be a significant improvement. The current offerings make it difficult to have a good local wireless signal distribution due to demand on gateway placement to improve the cellular reception. It is possible to add external antennas to the gateway but there is risk involved and additional technical hurdles to overcome. It would really be nice if they offered a gateway with the external connections. Options are always nice to have.