Im trying to approach this systematically.
Not having surface features (or the few it has being geological time stable, few, and/or eroded), all suggest a very old planet. The main driver is internal heat, both from formation and from radioactives. Earthlike planets as far as we know, form by aggregation and collision - meaning immense heat. Extreme age - or perhaps unusual composition - could maybe explain that. Also probably explains its flatness a lot (time for erosion, moonlike "seas" from impacts, no or minimal "rebuilding" dynamics like tectonics, water erosion when those re-melt and begin to move), which is helpful.
Second problem. Earth's magnetic field protects it from the solar wind. If its that cool, there probably isnt a rotating metallic core. (There could be a small one, but if so, not magnetically significant, or else I guess its hot and moving enough for tectonics). So no meaningful magnetic field.
So then the next issue, is, explaining an earthlike planet being "that old". Old enough to lose most internal heat and have a solid core (probably or for practical purposes) yet still support an earthlike environment over geological time.
My guess is it formed long long ago, long enough its been ejected into interstellar space when its star died or by some near collision. By some handwave it ends up in a near circular ideal orbit around another star some 10 billion years later. (woooo!). Perhaps its much smaller than earth - hence denser. Iron higher percentage maybe? Could help it lose heat.
Now in orbit, it slowly warms - but from the top down. Its still essentially solid and cool inside. (Note that cool just means "cool enough not to have moving liquid internally, even with the immense pressure". Not absolute-zero cold). Its orbit means heat is constant hence with no features or tectonics, so is climate over time. Pretty much. Within handwaving distance.
Bear in mind its already done a full evolutionary cycle. Its oxidised its iron the mandatory 2 billion years, got water and oxygen, got complex molecules etc. Theyre all there. Since complex chemistry can occur in space, these wont break down to the point that everything has to start from scratch. As son as it warms a bit, it'll have its liquid water, atmospheric oxygen etc. Or could have.
Now you have some much easier problems left. The (re?) evolution of life without magnetic fields, but with that helpful starting point. Maybe it never quite died out. After all, life can handle 0 - 400 C (ice water to deep sea vents) on earth. And a cooled planet only means under 1000 C (liquid iron/rock) or some such. So perhaps as the planet cooled, theres going to be a band within the planet that is a suitable temperature for simple life. As that shrinks, life retreats. Tricky, if it retreats too much, surface warming cant restore it. Maybe it didnt need to and it re evolved or came from elsewhere.
But id say something like that is your probable best bet.