Thanks for reminding me. If a mobile basestation is not usable, it seems to me that the best way is trying a WWVB-disciplined oscillator. Shortwave is certainly not available indoor, but longwave reception is possible. Hopefully it won't be killed by NIST due to the funding issues...
I’ve never been successful getting long wave reception in a data center (didn’t try running cable to an external antenna though, as then why not GNSS if that’s doable).
That and also many DCs I’ve been in barely get cell signal on the floor.