I have some 'regular router + dongle' experience and it is not positive.
Modern dongles tend to act like routers themselves, so you often end up doing some sort of double NAT. You can kick some of them into 'modem mode', but that can be difficult (I had to break mine open and short some pins to enter a firmware flashing mode, download sketchy stuff online etc etc).
Once that was working, it was crashing anyway. I have tried with a few models of router, all around the £150 mark. Perhaps pricier kit might work better, I suspect the weak link is the dongle, not the router.
When it worked I never got anywhere near 4G speeds. I have always ended up just using USB tethering on a mobile.
I am curious to hear others' experiences, especially with built-in SIM solutions. My current ADSL has an upload speed so slow it gets crippled every time someone gets on a VPN. In the current situation this is not great...
