Windows phone account and achievement syncing problems

In light of the recent announcement of the closure of Windows Phone/Mobile functionality, I thought I'd try and get some of my old devices out and get back to some of the older games I hadn't finished. However, I just seem to have been met with problem after problem, and I'm semi-convinced there is some strange account error going on in the back end which makes everything I have tried utterly pointless. To summarise;

I have 4 devices - 2 from back around 2015, 2 I bought recently. An original Lumia 920 running WP8, a Lumia 735 now upgraded to Windows 10 Mobile but which was previously WP8 and 2 new phones, a Lumia 650 and 550, both running W10M.

My account is otherwise fine, I can sign in to xbox.com, smartglass, windows store, Xbox 360/One/One X/Series X. I have checked and all the games were purchased with this account (I have the receipts in my inbox) and the correct email address.

Problem 1 - on the two newer devices, I get a 'sorry, we can't sign you in. please sign in at xbox.com then start this app again' error message on every game I start up. I took off 2FA and it made no difference, and had made a shorter password already to allow getting in to the X360 console. I've never been able to get past this on the two newer phones, though if I go into Airplane Mode it does avoid the error. This leads me to think that there is a check going on against my account and for whatever reason it is failing.

Problem 2 - on the two older devices, i.e. phones I was using back around 2014-2015, these errors never show up. Instead, if an achievement pops on the W10M device I do get the banner and it sits as a notification in the Xbox Console Companion App, but then it never syncs to my profile. For the WP8 device it pops in game, but then again goes nowhere. I've waited the 72hours etc and nothing ever shows up. I have noticed that in Halo Spartan Strike, I usually get an 'not connected to Xbox Live' error, although it still has my account loaded. I'm wondering if the only reason it has my account is from a local version or something similar, which would explain why older 2 phones work and the new ones which never had my account don't.

I'm highly inclined to believe for whatever reason there is some problem with my account - but I don't know what. I've tried going through every setting I can find and nothing seems obvious, and I'm all out of ideas. Does anyone have any thoughts on what I can do, or am I stuffed at this point?

:)