I had a hard time finding information on dummy loads for wireless antennas. Can you please give me (and others) a short primer and/or the right keywords to search for? Thank you.
(I was looking for my "smart" TV, I ended up disconnecting the wireless card from the TV. Thankfully the error message isn't as annoying as other models.)
You basically need a resistor with the value of the impedance of the antenna, and a wattage and airflow sufficient to dissipate whatever power is being dumped into it (or several resistors in parallel whose heat dissipation adds up to the value you need).
There's no real standard for cellular antennas as there is for many other types of radios (as far as I know, not that I would / can find in some brief looking). Most stuff I'm familiar is 50ohm or 75ohm. E.g., wifi antennas are generally 50ohm. If you made me pick a resistor to put on there without being able to test anything first, I'd pick a 50ohm/5W. But you might burn your electronics up.
So your two main unknowns are the impedance and wattage.
Solving for wattage is probably best just done with Google. The power output should be fairly standardized. Assume the transmitter's going to go into its highest power mode when it realizes it can't find anyone to talk to and then leave yourself some headroom on top of that. On a very brief look, most sources put LTE at about a half a watt, but other techs at up to 3-4W. For the price of resistors and assuming these will end up mounted somewhere without great airflow, I'd probably just throw two 5W resistors at it.
Solving for impedance is going to be a bit more challenging. If you're lucky, an off-the-shelf antenna _may_ be marked with the frequency ranges and impedance. Otherwise, measuring impedance generally requires some more specialized tools. You could pick up something like the NanoVNA2 to do the measurements yourself (they're not the best, but likely close enough for what you're doing and cheap relative to the other options), or try getting in touch with your local ham radio group and ask if anyone has an antenna analyzer to analyze a cellular antenna (impedance is at a given frequency, so they need an analyzer that covers up to at least around the 1GHz range, which are less common than one for HF which is only covering up to a few dozen MHz) and could analyze an antenna for you.
Then just hop on Digikey/Mouser/etc, grab some resistors and a SMA or whatever other connector would plug in in place of the antenna and get to soldering.
Disclaimer: Amateur radio guy, not RF engineer. It's been years since I actually used any of this information in any significant way. This could all be entirely wrong.
(I was looking for my "smart" TV, I ended up disconnecting the wireless card from the TV. Thankfully the error message isn't as annoying as other models.)