Precise point positioning (PPP) has been a competitive global navigation satellite system (GNSS) technique for time and frequency transfer. Classical PPP is usually based on the ionosphere-free combination of dual-frequency observations, limiting flexibility in multi-frequency scenarios; additionally, unknown integer ambiguities are not restored to integer nature, underusing high-precision carrier phase observations. Using undifferenced and uncombined (UDUC) observations, this contribution derives a time and frequency transfer model suitable for multi-constellation and multi-frequency scenarios. For short and medium baselines, ionosphere-fixed and ionosphere-weighted UDUC models are derived using single-differenced ionospheric constraints. Ambiguities are solved in double-differenced form and restored to integers. GPS data from several time laboratories show that models with IAR improve frequency stability by 25%-60% and 9%-30% at averaging times from tens of seconds to one day for short and medium baselines, respectively. For long baselines, the UDUC model is 10%-25% more stable than PPP for averaging times below a few thousand seconds and over one day.