The first thing is to determine what network service you need.
- is it while you are on the road or at stopovers
- primarily voice, data or both
- if stopovers are they near civilisation or wild camping
- how isolated are the areas that you will be visiting
- what is the realistic speed required (on road and stationary)
- what other devices will use the network
The most reliable network is EE but travelling around many very rural areas this can easily break and there will always be locations without any useable signal.
In rural Wales some locations only had O2 and this could be 3G.
My experience is that you really need a Dual SIM solution in a Primary/Secondary arrangement. Ideally with the two other network options available by SIM swaps. The actual pairing will depend on your use case for instance O2 are good for voice (availability of 3G or 4G not quality) so you can have a simple arrangement of an O2 (MVNO such as Sky) for voice and data on say an EE SIM. I check general coverage before any tour. To save going to different maps I use msh's
https://bidb.uk/ and hover for the mobile provider to pick the best pairing.
I haven't really bothered with antennas as I have had poor results, wasted a lot of money and I do not want to spend time playing with them (not travelling alone). Instead, I prefer allowing the device to get the best of whatever the mast I come across can provide. For touring my roof is fibreglass and I have successful results with tethered phones, MIFI and Routers. When on site I have a 5m telescopic pole that I can hoist a MIFI on (tried 5G phone but too expensive to risk).
On my last visit to Wales, I stayed just outside Brecon and had excellent 5G on n78. 5G may become more significant going forward and of course compatibility with n28. In the Brecon Beacons I lost all signal and GPS and had to just drive by the sun to the next main road. It is how it is.
I have used the following:
Note : When using good phones to protect the batteries I use a Chargie and its app. The phones have a USB 2.0 speed limitation. I have tested my iPhone 13 but do not use it for tethering. In addition, I do not use WIFI on tethered phones and shut down as much as possible to reduce charge cycles and heat (especially in summer).
Two tethered 5G phones (Samsung A22 5G and Moto G50 5G) using Ethernet to Edgerouter X (load balancing) to various makes of WIFI APs.
One tethered 5G phone directly to GL.INET Beryl via USB
Two tethered 5G phone directly to GL.INET Beryl one on USB the other on Ethernet. The Beryl set for failover.
Two tethered Cat 7 MIFI (old BT Mini and EE Mini Cat 7). One via USB to the Beryl and the other via a Mango to the Beryl. Beryl in failover.
Currently playing with a GL.Inet Opal as a cheaper 4G option, Openwrt and load balancing.
For mast testing I use the 5G phones but for 4G I use a single Alcatel HH71 (Cat 7) and more recently a Zyxel NR5103E (excellent but big) but these are not really suited for general van use as they cannot be mounted high in van roof for their reception and be unintrusive.
In addition to tablet etc I have my personal iPhone 13 which can connect with the van WIFI. I also have an old iPhone SE on the dash with its own SIM (low data plan) on the dash to stream (radio and music) and for GPS. If a mobile signal feeding the van is lost the van WIFI will still be present so to provide resilience whilst on the road I use Speedify software so the switching between the van WIFI and the internal phone SIM is instantaneous. Breaks in streaming are very rare.
The combinations are endless. You can of course go and buy a single unit to do it all, but I always set myself a budget for a solution.
I steer clear of cheap 4G Cat4 stuff now (other than what I have already) as these are too expensive for what they are and less likely to get the best from a mast now. Hence use of Cat 7 MIFI which are about £30 on eBay and easily unlocked. Will use 5G modems when cost/availability comes.
If you are to be parking in very remote car parking areas, you are very likely to have no signal and may be on shared masts at others. Satellite is probably your only option. Cheaper options are on the horizon but for now it is an expensive investment and £50 per month. Yes, you will probably need a generator for prolonged visits (assume you will have solar panel).
Edit: Just thought. You also need to carry a WIFI device you can put into Client/Wisp mode as some locations may have WIFI pubs/villages/camp sites.