This thread is posted to try and add some actual usable information, beyond the standard Plusnet response to roaming issues. I hope that information like this will be added to the roaming FAQ if there is a good response!
Roaming appears to be totally unusable in Bolivia.
One phone tested (Samsung S6 Edge Plus). It won't connect to any network so far in Bolivia, saying "Emergency Calls Only". This may sound like a remote place but there is an extremely strong presence of multiple mobile companies providing complete coverage at least in the towns.
SMS: Can neither send nor receive.
Phone: Can neither send nor receive.
Tested in Uyuni (so far).
The same phone connects immediately with a stable, fast data connection with a local SIM. This is not a coverage problem.
Please add any more information or fixes!