My understanding is the signal received at the external antenna gets to the internal antenna via cables (and the repeater). Not via wireless.
I've been involved in a few hundred installations of cellular amps. You're not following what the problem is. Forget what you think is happening. Consider this...
The inside antenna is close to your phone. It picks up the normal cellular signals from your cellular equipment inside that room. There's no change to your phone - it works like normal.
The amp then amplifies that signal and sends it out the outside antenna. And you're right, those antennas are wired or cabled or whatever you want to use to describe it. That is all how it is supposed to work.
Now think about this...
If the inside antenna and the outside antenna are too "close," then the radio frequencies being sent out the outside antenna to the towers are also received by the inside antenna. It's just radio and unless that inside antenna is isolated in some way, it will "hear" the amplified frequencies being sent out the outside antenna. Isolation happens by materials (steel) or distance.
Since anything received by the inside antenna is amplified again, it creates like radio-amplification-loop which within a couple of milliseconds would blow out the amp because the signal gets amplified over and over greatly increasing the signal amplitude through each loop. This is no different than the screech heard when you put a microphone in front of a large speaker. The slightest sound is amplified over-and-over and the result is an incredible overdrive.
Cellular amps are made to detect this overdrive - it's easy because the signal strength gets above a threshold. When that happens, the amp automatically turns down the gain. That's just like the microphone example where turning down the volume will make the screech go away (or moving the mic further from the speaker).
On a fiberglass boat with the two antennas within about 45 feet of each other, the gain is reduced so much that there is no amplification being made. At about 45 feet of separation (tough to do on a trawler) there will be some amplification. At 90 feet of separation, the amplification will work pretty well. Otherwise, it's better to just go outside with the phone and stand higher. This is the reason why people installing wireless amps on boats find that they don't work. The cel fi go is a wireless amp.
Now consider a cradle used instead of the inside antenna. That cradle is designed to have a few inches of range. In needs the phone to be in direct contact. It can't "hear" the output from the outside antenna. The result is a system that actually works. The nice thing is that these wired/cradle amplifiers are significantly less expensive. Usually about $180 vs $500+.
So the key, like the microphone example, is getting separation between the inside antenna and the outside antenna. A steel boat will give that although even there, placement is important because boats have windows and other mechanisms that allow signals to slip inside. Cars have a full sheet metal roof, enough to provide good separation between the inside and outside antennas even when there is only a few feet of separation. Put one of these amps on a fiberglass Corvette, and it won't work either.
So there's nothing special about the cel fi or any reason to dislike their specific products. It's the entire class that won't work with most boats. You'll install it and convince yourself that you're seeing a couple of extra bars. But be honest with yourself and you'll see. There'll be no added amplification and you'll get no better range with the phone. Again, it's not the cel fi product. It's physics. They're happy to have unknowing people buy their $500 product because they won't really be able to tell if it's working or not. These manufacturers also have no understanding about boats and their material composition. If it works in a car, why not a boat, right?
Before it comes, carefully measure the real distance between where the inside and outside antennas will be on your boat. See if you can even get 45 feet - probably not. Call cel fi and ask for technical support and ask them specifically what will happen if the two antennas have the separation you measure without a steel roof. They'll probably tell you that you need 40 feet - and that's about the minimum for a 50 dB antenna to get some limited amplification (where the auto-gain reduction starts to allow 5% of the amplification to go through). They claim 100 dB which is a stretch that I don't actually believe although Australian laws might well be different than US laws.
Save the packaging or just reject the package when delivered and hope for a full refund. Or keep it all and hopefully learn the $500 lesson it will provide.
