Working next to a seawall and under a dock was a learning experience.
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Cool vehicle!
You think the cellular link would improve if you move up that Huawei modem a bit higher, like using a simple usb extension cable?
I currently have it on a 12" USB Cable high in the boat. This was the first problem I have had.
Got it David.
Just trying to get used to the idea that you are sitting inside your home in front of your pc and getting full control and video link to your vehicle.
Where I live, in Beijing, cellular mast are not scarce at all 
on the contrary, the whole subway system and all elevators and garages in the city has full 4G coverage!
If I put your gear on my traxxas car I could drive around in FPV all over and come back.
Maybe I will just try that.
We don’t have scarce 4G coverage really. I have bench tested this extensively inside my house 1/2 a KM away with no problems. I think it has something to do with the location of the towers and the fact that I was operating on the coast and 4 meters below the surrounding ground and only have towers to the East of me. Once I was 10 meters away from overhead environment I was working under everything was good.
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The cell towers’ rays are bouncing and some penetrating the water. The see wall roof is a big path loss obstacle. And being in between the ceiling of these constructions and water maybe challenging for further path loss. Thus the power of rx is falling under a desired min power, causing instabilities in your fpv application.
This is a one interesting scenario. I am not an expert on telco and network models but if it only happens under those sea walls you may have to come up with a solution that changes the placement of your cell receiver.
For example : a floating waterproof tail like structure that doesn’t get covered under the sea walls etc. 
Thanks for your input. I went into this job knowing that I could have problems and had a few back up plans. I will say UAVcast in most situations is fantastic. And yes, It is one interesting scenario that was pushing the limits of the hardware and software. Ardupilot did what it was supposed to do by doing a smart rtl but something was in the way. Had it gotten out of the over head dock situation taking back over the vehicle would have been no problem with out having to switch over to the RFD 900 unit.
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I’ve changed some data rates and am going back to the same seawall tomorrow to do a follow up on the work that was performed. One problem might be that my Pi 3B+ had its WiFi on and could have been causing interference. There is a sudo command to disable it after I boot up and I will be using that also tomorrow.
Yes that may be the issue as well. Looking forward to see the results.
Btw are you interested in solar powered models? I have some long service time autonomous vehicles ideas on my mind
@David_Boulanger, How did your test go?
I was just thinking, your Huawei Lte usb modem got two connectors for external antennas right? I just learned there are several kinds available.
Maybe it can improve connection quality…
It went good. I have the optional antennas but I am not using them at the moment. I went with a telemetry rate of 57600 and lowered the bit rate of video way down. Also disabled the WiFi on the Pi after booting up. This cuts the ram usage on the Pi way down and probably was interfering with the modem. It still was a bit more latent when under the dock but never to the point of invoking a failsafe.
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I also don’t know if the water is reflecting signals is causing problems. Although in open water I have little issue. I may be the only person using UAVcast on a boat. @Sinamics