I`m not an expert but have recently been in a very similar situation to you as in I`m semi rural with a mast on my doorstep and also have a NR5103E from eBay and a Smarty Unlimited SIM. Also Three say there is no 5G in my area.
After getting all the kit setup, it didn`t take long to find out that the mast I can see from the bedroom window wasn`t going to give me the throughput I wanted and I was getting similar speeds as yourself, including the faster upload than down. If anything the local mast is a hinderance preventing me connecting to masts behind it.
The internal antenna in that router is better than you might think, I found (surprisingly) that by sitting the router in the middle of the loft floor (elevation didn`t help in my case) and rotating it while monitoring the GUI (192.168.1.1) I could get reasonable connections 4 or 5 other masts up to 8 or 9 miles away, some that did offer 5g (n78). I spent some time in auto band mode, rotating the router with payg sim cards from all the big 4 and compiling a grid containing ISP, cell ID connected to, bands connected, RSRP and download/upload speeds. I settled on a mast around 6 miles to the west at around -105 with download between 300-400 and upload a stable 20. This mast is over the other side of a hill covered in large trees. However when driving past it, it is clearly a more substantial mast in comparison to the one on my door step.
I may have your location wrong but looks like there is a cluster of masts to the south in Matlock and maybe even looking East is worth a try if you can get past the local mast.