Can anyone help? Ethernet & Wi-fi speed issues

[cupid_stunt]

Hello all,

Apple Mac mini Core i5 2.5 (Mid-2011) MC816LL/A - Macmini5,2 - A1347 - 2442

Standard AirPort: 802.11a/b/g/n
Standard Ethernet: 10/100/1000Base-T
OSX 10.13.5 High Sierra


I've upgraded my internet to 500mps (Virgin Media)

I have checked with various speedtest sites and my speed can be anywhere between
56-112mps over Wifi.
I checked my macmini using my granssons ethernet cable and am only getting 269mps.

***My grandson is getting well in excesss of 500mps on his windows 10 pc using the same cable***

Virgin say that they guarantee at least 400-450mps.

I have tried setting the MTU (after checking it in terminal) to 1500 (1472+28)
On the virgin media router. No change. Still the same lousy speed.
After restarting the router etc.

The funny thing is I also have an old G5 macbook that is 100+ on the mac OS side but 250+
on the partitioned installed windows 7 side !

Can anyone give me any more pointers as how to speed it up?
I do realise it is an older macmini, but I would have thought I could get faster speeds
than I am currently getting.

Thanks very much in advance for your help.