Do you use the same SIM card in both phones to check, rather than 2 different EE SIMS as EE offer a number of plans... some of which have data throttling.
Like yourself, I too am on the EE network and I have a fastest speed plan with unlimited 5G/LTE data minutes and texts. Since 5G has yet to arrive here in the Cotswolds, I too use 4G/LTE. My download speeds do fluctuate throughout the day but are around 90-140Mbps.
Here is my current test on my Galaxy S21 Ultra on EE, using Ookla Speedtest, free from the Play Store...
It might be worth contacting EE's customer services to receive an explanation or investigation .