The gain on a bus roof mounted antenna is not likely to be that significant, given a bus has plenty of windows. If there's poor signal outside then it's likely going to be poor for the bus Wi-Fi too.
I don't think buses have multiple SIMs. If they've chosen a network with better coverage in a particular area over yours, that might help, but I'd expect bus companies negotiate a rate and get multiple SIMs from a single network and don't look at coverage maps.
In rural areas, the bigger problem will be if you drop to 3G. Irrespective of the coverage level, most networks have refarmed most of the spectrum to 4G and so there's very little bandwidth to go around. And something called cell breathing can destroy coverage one day over another, because of demand (which has increased due to less spectrum).
Some handsets can hand back to 4G from 3G quite quickly. EE configures its network to facilitate this even when data is flowing. On some networks (and equipment) once you fall to 3G it stays there until there's a period of inactivity (which there won't be if lots of users are trying to access data). If the bus system can't revert to 4G (assuming it even supports 4G) then it will be stuck on 3G and you'll often have no data flowing at all.
4G and 5G will eliminate most of these issues, massively boosting rural coverage in the process, which sort of makes Wi-Fi even less necessary!
