Well there’s good news and there’s bad news. 
The good news is that I now completely understand the problem and I am able to explain, reproduce & rectify it.
Details
Many moons ago I decided to archive my ogs account and start a fresh one so Kosh (on OGS) became Kosh.2017 and a new Kosh was born. Now for over 18 months all was well because I never logged out of forums and was only using the same device. Unbeknownst to me, the new Kosh was/is not actually linked to my forums account. When I clicked on ‘Forums’ it simply took me to forums where I was already logged in without reference to what OGS account I was using.
When I tried to use a new device and clicked on ‘Forums’, it automatically logged me in based on the open OGS account which caused it to create a new forums account. I was able to rectify this by logging out of forums & OGS, logging into the old account (Kosh.2017), clicking ‘Forums’ and then doing as I please on OGS. So I can now access my forums account from my phone. yay!
Regrettably that brings me to the bad news…
The bad news is that I’ve fallen foul of another problem while trying to sort out the first problem, so for now I’m ‘Kosh.2017’. If you don’t believe I’m me, @kickaha, just check past posts. 
Details
Chapter 2: (Because nothing to do with computers should be easy.)
When I used my archived account to correctly log into forums, the system helpfully (
+
) did an automatic update of my forums name which is why I now appear as Kosh.2017. sigh. I can eventually fix this by giving my current OGS account a temporary name, renaming the archived account ‘Kosh’, logging out and back into forums, waiting 3 months and then restoring my OGS accounts names. You can all laugh now.