I have just posted in another thread regarding choice of indoor turbo-trainer, to dissuade people against parting with a lot of cash for a static indoor bike that has limited adjustments, limited comfort possibilities and is expensive:
https://www.cyclechat.net/threads/is-a-wattbike-worth-the-money.263779/page-3#post-6225956
I would recommend you a used Tacx Neo original. It's model number is 2800. You will find many for sale on
ebay at high 'buy it now' prices, because they are solidly built and reliable. Age gets to us all and fixing it is easy with a simple tool. In my nearly 4 years I've never had to fix or do anything to it except give it a wipe down now and again with a cloth.
You can use it without power, but if you stop pedaling the ANT+ and bluetooth connection will of course stop working. I do this to encourage me not to stop cycling and have never had a problem maintaining connection to a laptop, phone or tablet using an ANT+ dongle or bluetooth. I prefer using ANT+ as this means my bluetooth headphones for tv/podcast/music/whatever won't interfere on the bluetooth channels.
If you're going to spend £600 on a new mid-range turbo trainer then, for my money, I would favour the premium range used option of a Tacx Neo version1 for around £750 or so. One of the best features for me is how quiet it is. Mid-range turbo trainers are noisy whereas the Neo is relatively silent, you just hear the sound of your bike groupset moving.
Features that won't work without power are:
'road feel' - this really does make your bike feel as if it's on cobbles/wood/dirt but I find it to be a fad and don't miss it.
Connectivity: after around 1 to 2 minutes, as I mentioned, your device will disconnect from your computer/tablet/phone because the ANT+/Bluetooth switches off without mains power IF you stop pedalling. However just pedal a few times and it'll come back on.
Having used many mid-range to upper range turbo trainers I cannot recommend the original neo enough. Is it perfect? There are some reports of power reading inaccuracy, but we're talking 10watts of difference or so, so nothing crazy. Otherwise, they appear to me to be bombproof. I avoided the new 2T model to get an older used model, due to the numerous reported problems with the 2T.