I have an LTAP Mini mounted in my vehicle. I'm wondering how much of a difference it would make if I got an antenna outside the vehicle.

My options to do it would be:

Some kind of window-mounted sticker type

Using the window-mounted sticker type, but mounting it inside the shark-fin type of antenna holder the car has on its roof (currently used for the AM/FM radio).

Mounting a puck-type antenna on the trunk.

Has anybody done an in-vehicle LTE router/modem before? Did some kind of external antenna make a difference?

I work in a city radio shop. We have hundreds of vehicles with various radio services - GPS, Wifi, cellular, and of course the two way radios. I can absolutely assure you that getting the antenna outside the steel box known as your car will SUBSTANTIALLY improve radio performance. Best bet is an antenna that is mounted on the roof with a hole drilled through the roof for mounting and routing the cable or cables. Likely the next best is a mag-mount on the roof - be careful not to damage the cable getting it outside. Through glass antennas are less effective than an antenna connected directly with coax cable. Any of these will beat an antenna inside the car however.

I have an LTAP Mini mounted in my vehicle. I'm wondering how much of a difference it would make if I got an antenna outside the vehicle.

My options to do it would be:

Some kind of window-mounted sticker type

Using the window-mounted sticker type, but mounting it inside the shark-fin type of antenna holder the car has on its roof (currently used for the AM/FM radio).

Mounting a puck-type antenna on the trunk.

Has anybody done an in-vehicle LTE router/modem before? Did some kind of external antenna make a difference?

I'd like to re-purpose the "shark-fin" style antenna enclosure that's already on the roof of the car if I can. That way I don't have to come up with another antenna mounting locaiton.

Do you know if there is a small "sticker" type antenna that works for LTE? For example, laptops typically have a piece of copper(?) foil that is mounted up against the chassis of the laptop.

Another thing I guess I could try is to take apart a "puck" type antenna and see if I can't use some type of adhesive/epoxy to affix that to the existing antenna enclosure.

I'd like to re-purpose the "shark-fin" style antenna enclosure that's already on the roof of the car if I can. That way I don't have to come up with another antenna mounting locaiton.

If you don't mind spending some money, you can get custom (or semi-custom) antennas that replace the factory shark-fin antenna that includes additional bands. For the microwave bands, that is quite easy since those antennas are small. For UHF and even more so VHF, it is harder, but can be done.