NOT using CGNAT - using Three with the 3internet apn which gives a public IP address.

As part of my endeavours to find a solution I did use the CGNAT more general apn three.co.uk and found no difference- except I lost the public IP.

I think part of the problem might be a fairly regular (say 50% of the time) loss of upload: it is often <1mb/s and can on some tests at certain times be zero! I have tried 3 customer services who promised a call back that didn't arrive. Not sure calling their tech support is going to be much help though: I believe they will just tell me "upload is slower than download" and, if we get technical, "only uses 25% of available spectrum" and "we only usually say you will get upload approx 10% of download" etc etc. An example of this in the early hours of the morning when there is little contention I can get 70-80 down fairly consistently, yet the upload still stubbornly, more often than not, is <1mb/s. That's part of the reason for balancing with my ADSL at least with this method I can almost guarantee some upload!

Read the article link posted (thanks for that); I think I've read it before. Seems to suggest there is not a technological reason from Sky's perspective why it shouldn't work.

I'd like to try a different providers SIM but for a number of reasons, that I won't list here, that's not easy.

Be great if I could at least confirm from another user (of whatever 4G router) they have it running and what settings they used to try to "rule out" router from enquiries

I had Three mobile for a year and never got Sky Q to work properly (I know you said Sky+ but it same result), never figured out why, same problems you getting exactly.
No idea what it was, as soon as my FTTP connection went live all works, no router changes as I was using 3910 before and after, literally just setup the new wan connection.
TBH Three at the end was shocking, DNS delays, sites not loading, mega slowdowns - when i first got it it was great.

And.....although it is still failing...I have found it more consistently starts the download on 3G (another issue not sorted- I get better upload but obviously a hit to download). Odd, the red led for recording now comes on fairly consistently and the download similarly starts. Always failing at 5-6% of download.

I still wish I had a VPN on the router to eliminate any form of IP/Server management by Sky for public IP's belonging to mobile companies....and to try UDP, which I have found with ADSL provider can sometimes be detected and blocked presumably by some form of packet sniffing or "pattern" to the download, whereas TCP is more "normal" traffic and less likely to suffer this fate.

In your endeavours did you try a VPN by any chance? Or are there any proxy settings on Draytek I could use to "spoof" an different IP?

Rolandrat, I suspect when you plugged in your new line the default IGMP settings were applied; as far as I can tell these settings cannot be applied to LTE line.

What are your IGMP settings on the "new" WAN?

I'm wondering whether I might route via ADSL that I know works and then try enabling/disabling IGMP settings to prove whether they have any influence....

Did you try with a router-based VPN by chance? Another thought is Sky blocking mobile companies servers/public IP's or preferring TCP to UDP etc?

I doubt it was any router config setting as in that time with the Three Mobile connection I have used a Synology RT2600AC, Draytek 3220 & 2960 & 3900, Unifi UDM Pro, Unifi USG and my current Draytek 3910.
All never worked other than start and fail or just fail.

I never tried a VPN on the router.
I also got them to remove the "Go Binge" as that slowed down Netflix in the last few months - like I said towards the end it was a constant battle, totally useless, Im so glad I just managed to sneak a FTTP install in a week into the Covid lockdown by doing all internal installation myself eg drilling holes, install ONT etc.

Thanks rolandrat.

Odd though isn't it that even if we cannot solve the problem I haven't found anything across the internet/forums etc speculating as to the likely cause despite all the expertise out there. Someone must at least have an explanation even if no solution

I'm waiting FTTP too but with company with awful reputation and delays even before Covid; it was supposed to be "second quarter of 2020": we're nearly there now and heard nothing.