I did a check for mobile coverage and 4G was good inside and outside. The device management software is showing a good signal but the upload and download speeds are pretty poor. I am wondering if the actual mobile infrastructure is at fault and cannot cope with the demand?
As you've found, signal level doesn't necessarily mean good speed with mobile (whereas with something like WiFi it generally does). Also, what are you checking to see the signal level? If its the number of 'bars' you have, those are an interpretation of the signal metrics: RSRP (power), RSRQ (quality), SINR (signal to interference + noise ratio).
Those individual metrics give more detailed information on what signal you are receiving and potentially identify what changes might lead to improved metrics. If you look at and note down those values over time when you're experiencing the variations in speeds I would
think they show some kind of pattern.
There could be a number of reasons for low speeds.
Overloaded capacity on the antenna/mast (too many users) is certainly one, however you say you're rural, so that might well not be the case.
The speed of the back-haul connection to the mast might be another, though I see this mentioned quite a lot, but I believe in most cases it's generally not really a limiting factor, except in the cases of multiple daisy-chained microwave-linked rural masts - which might be the case for you.
Finally, you say you've future-proofed yourself with 5G, however I don't believe
any of the operators are planning to deploy 5G as far as they currently have 4G deployed.
Personally I think in the mid-term timeframe (up to 5-8years?) 5G will just be focused on urban/suburban deployments, where it brings the most benefit/ROI - I really wouldn't expect rural areas to get 5G. Though obviously there might be exceptions to this where rural capacity could be needed - I'm thinking places like center parks/theme parks/glastonbury festival site.