I have just started using vfe 4g broadband (actually an unlimited voice and data sim). I can't get zip on my 4g Android phone (Google pixel 2XL) at home, I can get weak 3g.
With a cheapo Huawei home 4g router I could get 3 bars (out of 5) 4G and 5 bars 3g. Actual achievable speed will also depend on mast congestion. I am 4 km from mast without great line of sight. The Vodafone coverage map said I should get outdoors 4G but not indoors.
I have now installed a directional panel antenna and get 4 bars and about 30 to 40Mbps down.
It is a real crap shoot. I think if you do get a 4g signal on a phone then with a home 4g router you will get a reasonable signal. If the coverage maps say you can get 4g (even if a phone says no) then it might be worth a punt but if you can borrow some kit to try so much the better. If the coverage maps give no indication of coverage then it is probably not worth trying. The coverage maps are 'optimistic' I think. A typical home 4g router generally has higher gain antennas than a phone I believe. If you think your coverage might be poor it might be a wise move to make sure your 4g router has external antenna connections. As to what type of external antenna, Omni or directional, again it is a bit of trial and error and some educated guesswork. The Mastdata website might help identify what towers are about.
I believe I can only get 4g from one mast so I decided to use an external, directional antenna to point there. For 3g I actually do better on the internal Omni directional antennas connecting to masts in other directions. This is a useful fallback if the 4g mast goes down.
I do think it is a bit trial and error of you are in a weak signal area.
I don't believe there is any real way to test connectivity workout a SIM. I took a gamble but you might be able to borrow a Three SIM and 4G router maybe?