Without wishing to add to your frustrations, the internet speed and what you get over WiFi are connected but separate. If you can use a cable from a laptop to the router, a speed test on that will tell you what's coming down the wire into the house.
WiFi is radio, so prone to interference and coverage limitations, and some routers are better than others. Interference can come from neighbours using the same channel - most routers now auto-select a clear channel but it's worth checking with a phone App - but also from multiples WiFi devices in the same house. You might find that any one or two lightbulbs will work but not all of them. Lower frequencies are better at going through walls, so if you can get the 2.4g back on that might help. Location of the hub can also make a difference, and you can get devices that extend WiFi range using the buildings main circuits to carry it between rooms.
Just to confirm - you can use both frequencies at the same time. EE support told me to work out which device worked best on which frequency and set and leave them. I leave TV and Ring cameras on 5ghz as they are fixed in place and fairly near the router. Ipads and phones that move around and my printer which is quite a way away are on the 2.5ghz
1 x SONOFF Mini Two Way Smart Switch. This is a mini and compact DIY smart switch used for different kinds of switch boxes, even the smallest EU standard switch box. WIFI: IEEE 802.11 b/g/n 2.4GHz. -Ask devices to turn on/off just by a voice command.
It's a tiny module that can be located in/above the light fitting. The plus is that you just use an ordinary existing switch that will work as normal, but it makes the light smart. The double plus is that it can work with 2-way, even an intermediately switched, (i.e. 3 or more switches controlling the same light) circuits.