No wifi calling support? I had thought that was nearly universal?

Also, IIRC, LTE is mutually authenticated and your parents would not be able to attach to a private LTE network unless they have a SIM that's setup for it.

I used to live on the US-Canadian border where the ATT signal consistently was outcompeted by Rogers and Bell, leading to my phone essentially never working. This was ~10 years ago and ATT provided us some hardware that provided a 3G connection locally when connected to our DSL connection.

Usually yes, but it is not an EU-wide system. Contact your spectrum regulator and ask for what options are available for private 5G^[1]. You can likely get a bit of spectrum in the 1.8GHz guard band, around 2.3GHz, or somewhere between 3.4-5GHz. Price is from free to thousands of euros, depending on the country, transmit power and whether you are using outdoor cells. A private mmWave cell will almost everywhere need to be under an experimental/R&D license (we don't have public mmWave networks).

If you can choose, it's best to take a TDD license, as the duplexer required for FDD base station operation is a relatively expensive device when operating on an uncommon band.

1) Private 4G wasn't really a big thing in EU, but in most countries the license you get is technology neutral and can be used with a 4G base station as well.

PS: Connecting the private LTE to the public telephone network with a publicly reachable number requires a public telecom operator permit in some countries. You will also need to pay for the block of phone numbers. Roaming to the public cellular network requires many extra things which are not cheap or easy to obtain. And of course you need to have a roaming agreement with the other operator.

Congress afaik relies on a temporarily borrowed/gifted license

probably not at present, depending on the size of the community - it looks like roaming between base stations was not solved.

What kind of (cheap) hardware can be used for that?

These FreedomFi units seem to only support CBRS frequencies.

Right now you can buy a licence which costs a nominal fee https://www.ofcom.org.uk/spectrum/frequencies/shared-access (£80 per year gets you 10Mhz in 2300, 3.8-4.2 GHZ, or 3.3 pair in 1800)