I love the smell of Brasso. Shiny copper pipes are the real thing 
Seriously though, a "travel router" style device with external ears antennas, will probably be the best way to go, making the weaker signal useable if positioned correctly. No need for the wall wart format in the boiler room, and I am guessing, no real need for the ultimate in performance if it is just the boiler monitoring/control system to be connected to it via ethernet.
another option would obviously be power lan, no wifi required (and also not delivered), but two power outlets. RJ45s in both ends.
take the advertised speeds with a big grain of salt, whatever they promise, divide it by at least two, or even more, and they'll still be perfect world lab values.
they won't run openwrt, but it wouldn't be a show stopper for most people.
I see the logic here and wondered about this myself. As strong as possible WiFi performance is hugely important.
Any device suggestions?
I tried one time but I think the signal has to traverse separate breakers at the consumer unit and it kept cutting out.
How much difference does AX make for areas where iPhone 16 shows circa -80 to -85 RSSI in places?
depends on the AX speed delivered, an AX1800 unit would probably match or be slower than AC2600.
how about https://www.amazon.co.uk/dp/B0D7PTFZZM ?
Really nice looking option. Specs any good?
I don’t mind paying more if I’d get better WiFi connectivity.
if you want to optimize it, you'd want a tri radio device, so the down link or uplink doesn't cap the 2nd radio:
Is the rest of the wireless network AX? Is 2.4GHz available (better penetration)?
@frollic's suggestion of a "power-lan" might be the best one if you want only one ethernet connection (depends on the wiring details in the building). Personally, I have used the gl-mt3000 for this, both wds and mesh at different times (hotel type environments), but there are other makes you could choose of course.
This is not good and will usually result in lots of retries, slowing things down considerably and adding random high latency. But this is just from an iphone, not a "router" with external antenna.
AX (at both ends) gives far better results as well (a bit outside the scope of the original question, but worth noting).
An rssi in the area of ~ -65dBm or better is something to aim for for reliable low-latency / high-speed traffic.
What is the rssi on the outside of the boiler room? Copper plumbing can kill 5GHz but let enough 2.4GHz through, but also creates multipath reflections (worse at 5GHz), so location is very important.
The dap-x1860 is a good device, but range is lower than on other devices.
Any suggestions?
How about:
That is a nice thing to aspire to, but in practice is rarely worth the effort.
The third radio is invariably used in the oem firmware to passively scan for other proprietary Easymesh devices (hence the numerous "crippled" third radios - usually crippled by not having proper antennas and a very limited power output for cheapness more than anything else.).
In the EU/UK (ETSI region) using two 5GHz channels is often very problematic if you want to avoid DFS.
AX aka wifi6, brings the advantage of using HE40 on 2.4GHz. It brings 2.4GHz performance up to scratch (~ 500 Mb/s between 2 stream radios (the cheap ones) and in excess of 1Gb/s on 4 stream (more expensive ones).
A lot of things to consider.....
Yes I’m not fussed about that. This is literally just to allow an Austrian Froling pellet boiler to access internet so I can control it and check pellet status etc from my phone. Signal outside (either from the room with the radiator pipes or outside outside) is much better say circa -65 RSSI. The nearest WiFi access points are a Zyxel NR7101 (outdoor wall mounted with AX) or indoor RT3200 (only AC). This is on 2.4GHz.
The ethernet over powerline didn’t work well back when I tried it (circuits are on different breakers at consumer unit). Maybe the technology has improved since I tried it a few years ago.
But I have feeling I’d be better persevering with WiFi.
Anyone have any comments on specs or experience of:
So far I like the look of this option a lot.
Can it run 24.10?
These are powerful and great value BUT do not have external antennas.
You are looking for penetration through boiler room walls. Might be ok for this use case... might not. Currently only on OpenWrt snapshot but that is not a problem if you use the firmware selector or equivalent for any upgrades until 25/26.xx is released.
You could hang one each side of the wall and link both back to your main router with a mesh, sounds a bit over the top though 
I would go with this because I have used them before:
... or the same hw from Cudy - TR3000, less flash and no active cooling.
Modern anti-surge breakers that are getting put into hotels and other public places do seem to dramatically attenuate the signal. On the same ring main is usually ok though. When it works it is certainly a good plug and play solution.
Which out of these various devices would you expect wifi connectivity to be best for?
I like your point @bluewavenet I could literally put two either side of wall if one didn’t work out so well.
Room is mega dusty. Does that device really have active cooling!?
I've read somewhere they don't work well when used between different electric phases (correct terminology ?), never tried it myself though (25 RJ45s in the house).