Regardless of the VPN provider you use, 1% signal is abnormal. Even though I have described the fallback behaviour of ZeroTier, that only accounts for latency increase (to the 100-200ms range). It didnt cause any severe telemetry signal degradion. Here are my suggestions ordered by most likely cause to least likely:
Check your actual data transfer speed. Use speedtest-cli or some other means to check what is you actual bandwidth available. Now this might sound too obvios, but here in Bulgaria when you sign up for a Data plan you get “unlimited internet” but only X GBs of data on max speed and then you fall bak to a few KB/s when you use up the quota. If that is the case you will still have connection, but it will be horrible.
Check your baud rate. Your raspberry Pi might not be coping well with too high baud rates (921000 for example). Yes, I know, you will say it is OK on the WiFi. But a lot of things are at play, maybe when you add the 4G module, then the raspberry Pi begins to struggle.
Check your telemetry update rates. If you have 100hz refresh rate on some of the telemetry groups you might be overwhelming the poor VPN connection over the 4G network.
I know this will sound sleazy of me, but I really think it might help you… My 4G telemetry product DroneMate is currently in open Beta. Try to install it and see if it will work well for you over 4G. If it works ok with DroneMate at least you will know it is some configuration you are doing wrong somewhere. If it is the same, then it might point to either a bad 4G stick or problems with your carrier. Link: https://drone-mate.com
P.S. I am still in testing phase of TailScale, while it does allow direct peer-to-peer connection between your drone and ground station (which is the ideal scenario), the jitter is all over the place. I get my lowest and highest ping times when I use TailScale which surprised me a lot. In addition it is supposed to be much more lightweight compared to using OpenVPN server, but in my tests the bandwidth I got from my drone to the ground station was about half of what I get with OpenVPN using AES-128-CBC cipher and SHA1 auth. ZeroTier is higher latency, but at least its consistent. Your mileage may vary.
P.P.S Take note if the 1% link indication is only while Mission Planner is downloading ArduPilot parameters list. If it goes back to reasonable levels afterwards it is a pretty good indicator that your link is simply saturated by the parameters transfer. If you are also streaming video from your drone (even if your Ground Station is not listening to it) it might be enough to cause your issues on a poor 4G connection.
Hi Thanks for very useful information.
I found out the reason.
The problem was with wifi data card. Am using Enter 4G data Card.
I have Connected my RPI into other network, then the signal increased to 99%.