iPhone SE (WiFi, no SIM card); "TextNow" app won't ring for incoming calls

My friend gave me his old iPhone SE. It works great without the SIM card over my WiFi and elsewhere. I've installed "TextNow," a calling/texting app. App works fine, but on my iPhone SE, it doesn't matter what notification sounds I specify for the app, there is no ringing for incoming calls/texts, making the phone next to useless for my purposes. (I typically work using desktops.)

The same app with the same phone number works fine on a much lesser smart phone, it rings when calls come in. Just not on this iPhone SE.

The phone seems to be "locked" to a carrier, even though there is no SIM card. I have seen videos using "unlock chips" for iPhones, but perhaps the problem is unrelated to that.

There is a native (I think) "Phone" app. It allows me to make calls through it using the app in question ("TextNow"). The phone does not allow me to uninstall its native Phone app, as far as I can tell. So, even though TextNow can work through that app, still, I will not hear the ringtone specified for "Phone" when calls come in for my TextNow number.

Please help. The goal is simply to have a WiFi phone that's usable at my house, various friends' houses, and other places with WiFi. Seems a shame to bag this iPhone when it is so good at making calls and texts with TextNow. But if calls don't ring when they come in, it's hardly a good phone.

Thanks for any tips! (P.S. I'm a total newbie to smart phones. Have never had one.)