Well, there are three ways it could go.
1. the CMB is what we think it is, a baby picture of the expanding universe
2. the CMB is the cosmological horizon in a non-expanding universe
3. the CMB isn't cosmic or background, it's the temperature of space around us
1. Is correct. It was the prediction thereof that caused the vast majority of former SS theory believers to jump ship, even before COBE, WMAP and Planck.
2. For which there is zero mechanism. Brilliant scientists such as Fred Hoyle went to their graves never having been able to explain the CMB in an SS or QSS universe. And, to the best of my knowledge, never even attempted to explain the ISW effect which says the universe is not only expanding, but doing so in an accelerated fashion. Then again, he died in 2001, and the best data on the ISW effect comes from WMAP and Planck, which post-dated his death. Again, to the best on my knowledge, no alternative model can explain the ISW effect in a non-expanding universe.
3. Errrr, no. As mentioned, it is cosmological beyond doubt. As the three effects mentioned prove. There is no explanation for those effects in any model which invokes a local source for the CMB. And there are no such models to start with. Unless you want to dive into the world of pseudoscience and crackpottery, such as the 'models' proposed by the likes of Lerner and Pierre-Marie Robitaille. Both of which fail horribly.
As for 'the temperature of space around us'! Well, why would it emit at 2.7 K in the microwave? At every distance? At that temperature everything would be neutrals. It quite obviously isn't.
And not to forget that any 'model' that does not invoke the BBT, must explain the light element abundance. Furthermore, any alternative 'model' will need to explain the predicted increase in the CMB temperature with distance, which is observed to fit the LCDM model. For instance;
Microwave background temperature at a redshift of 6.34 from H2O absorption
Riechers, D. A. et al (2022)
https://www.nature.com/articles/s41586-021-04294-5
If the CMB were actually local, does it have exclusive rights from the FCC to broadcast in its frequency? Or would other photons from deep space be mixed in?
That somehow shows a near-perfect BB spectrum? I think not!
And, lest we forget, you are ignoring (deliberately?) the integrated Sachs-Wolfe effect. That shows that the universe is not only expanding, but must be doing so in an accelerated fashion. Allied to SN 1a time-dilation measurements, and the BAO observations, that rather puts paid to any non-expanding and/or non-accelerating expanding universe. I have not seen any alternatives put forward to those combined observations, even from crackpots.