The primary oscillators include the External Crystal and Resonator modes, the
External RC modes, the External Clock modes and the internal oscillator block.

You set the "primary" oscillator with FOSC3:FOSC0 configuration bits - so with
the internal oscillator block set in CONFIG1H, and 8MHz or 4MHz selected in
OSCCON, you need System Clock Select bits SCS1:SCS0 set for "primary" for
PLL to work.

The PLL is available only when it's configured to use the internal oscillator
block as its "primary" clock source (FOSC3:FOSC0 = 1001 or 1000).

It's not exactly hit-you-in-the-face clear from looking at the datasheet, but
that's the only way to turn on PLL when using the internal osc.