From looking through your test results, it looks like the intermittent dips could be your 4G router switching between 4G bands. This is particularly an issue with Cat 4 routers that only connect to a single 4G band, such as the TP-Link MR200/400/6400 series and the Huawei B593 and B3xx series. For example, Three uses 4G band 20 extensively in rural areas due to its ability to penetrate building material, vegetation, etc. better than 4G bands 1 and 3, however, however they only have 1/3 the bandwidth on band 20 compared to band 3 and it's more congested due to users in fringe areas only being able to pick up that band indoors. Speed drops after around 6pm is mainly due to traffic load on the mast as the bandwidth is shared between everyone connected to the mast, like a road.
If you have a Huawei 4G router, you can try using LTE H-Monitor (link) to lock to individual bands (Configuration -> Radio). Choose a single Upload band and the matching download band (e.g. B3 1800MHz for both). Run a speed test, then repeat with a different band such as B1 and B28. If you get no signal, then that band is not in use. Three uses bands 1, 3, 20, 28 and 32 (download only). The TP-Link routers and most older Huawei routers lack support for bands 28 and 32.
Higher end routers such as the Huawei B628 and B818 can connect to multiple bands simultaneously (i.e. carrier aggregation) and also have Gigabit Ethernet ports, so can potentially deliver much faster, possibly 200-300Mbps if band 32 is on the mast you're connected to.