Convinced I had been sent a rubbish or faulty PoE injector I purchased a TP-LINK TL-POE160S https://www.amazon.co.uk/TL-PoE160S-Injector-Supplies-Wall-Mount-Distance/dp/B08LQP8CYD/ which immediately worked and at 1000baseT/Full. It’s also a much more professionally packaged product, sealed with manual and ethernet cable.
Link partner advertised link modes:
10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Amazon sent a Ver: 2.0 unit whereas according to TL-POE160S | PoE+ Injector | TP-Link United Kingdom the latest is Ver: 2.6.
I only installed it this morning but the RSBOOM and Raspberry Pi 4 Model B are powered and networked. Occasionally there is small packt loss, about 0.1% which I don’t think was there previously but this could also be because I’ve started dabbling with rsudp and added sending to a Datacast host (the “Shake live real-time data streaming and archived data access” downtime did have an advantage giving me the push I needed). I’ll keep an eye on the whole system and see how it behaves over the next few weeks. I notice sometimes ping times go up to 271 ms from say 5 ms so one possibility could be the unit is jumping between IEEE 802.3af 802.3at if the power consumption is on the borderline. It could also be because the injector is a Ver: 2.0 rather than Ver: 2.6 or possibly because this is just what PoE does.
So now I have my router and distant RSBOOM on the same UPS. Happy days 
!
Summary
REVODATA splitter https://www.amazon.co.uk/REVODATA-Splitter-IEEE802-3af-Adaptive-TYPEC0504G/dp/B0CHW5K5F4/ (5V/4A) and TP-LINK TL-POE160S https://www.amazon.co.uk/TL-PoE160S-Injector-Supplies-Wall-Mount-Distance/dp/B08LQP8CYD/ provide power and 1000baseT/Full but I have noticed occassional insignificant packet loss which might not be related.
Avoid PTZlink style PoE injectors, however my unit could have been faulty.