I pay EE so much as it is a mobile sim, and you can't get unlimited broadband from a mobile sim.
That's simply rubbish. Don't know who told you that.
I have unlimited mobile broadband from Three for £16 per month, from EE for £20 per month, and could have it from Vodafone for £35 per month, though I have a more limited one from Voxi for £10 per month. That's all the networks*
All these are mobile phone sims. All work just fine in a 4G router. Or in my case, three 4G routers.
There are regulations about Net Neutrality that prohibit limiting the use of phone sims in routers.
*O2 do it too, but in my experience O2 are too [censored] to bother with.
I end up buying 200gig a month on average, and the cost is £70 per 100gig. I have tried their home hub, which would have been significantly cheaper, but it couldn't get a signal.
Your best bet is to try a PAYG sim from each of the other networks to see what each gives you.
You'll need to look on Cellmapper to see which way to point the antenna.
Even then, it's not obvious. With Three, my nearest cell gives a good strong signal but it is overloaded and data speeds are very poor indeed, so I connect to a different one a few miles further away. The antenna is "hidden" behind a stone wall so it can't see the nearest base. Works very well, getting about 35Mb down, 30Mb up.
I did try a directional antenna, but the signal strength went off the scale and the signal quality plummeted.
With EE, it's easier. The nearest cell is the strongest and the fastest, and it is happy with the directional antenna. EE isn't as fast as Three here, but the latency is much, much better.