Help with 4G dongle (UK)

Having trouble getting a generic 4G dongle to have internet access. First tried with O2 SIM from my phone to test and it worked fine.

Then purchased a cheap Lebara SIM to use with the dongle but had no luck getting internet access after messing around with APN settings for ages. Contacted Lebara support whose response was that although some people have got their SIMs to work with dongles and routers, they are only designed to be used with phones. Lebara SIM cancelled and refunded.

Searched for other cheap SIMs and contacted the companies to ask if they would work with a dongle. Asda Mobile was one that their support said can be used with any device. So purchased that, but it also has no internet access in the dongle and haven't been able to get it to work after messing around with settings either. Contacted support who now are contradictory as they've said like Lebara their SIMs are only designed for phones even though some people can get them to work with other devices. They did try to help with APN settings but we had no success, and they said most of the routers/dongles people have had success with are Huawei ones.

I'm not sure if the issue is related to frequency band compatibility between the SIMs and the dongle. Not sure whether to try more SIMs or try to get the frequency band information to ensure compatibility... Or if I'd be better off getting a better dongle that might work with more SIMs.

Trying to keep costs down as it's only purpose was to provide permanent internet access to thermometers and cameras that can be checked occasionally from our phones when not at home, for pets safety. For other internet access we just use our phones data when we are at home.