I don’t know if this helps but it’s all I could find (quick cursory search admittedly):
Can. 276 §2 3°
I fully accept that I may be wrong on this matter. I am no expert nor even highly knowledgeable in this area. I have read the General Instruction on the Liturgy of the Hours (GILH) several times but it is a weighty document and every time I read it I ‘discover’ something new. My understanding/interpretation of a secular priest’s obligation is as follows:
(I have used the current names of the offices in the Liturgy of the Hours; my preference (as much as that’s relevant) is to call it the Divine Office and give the offices their ‘older’ names.)
1a.
Invitatory – mandatory if the first office he recites is the Office of Readings
1b.
Invitatory – optional if the first office he recites is Morning Prayer
2.
Morning Prayer
3.
Prayer during the Day – one occasion only: either mid-morning
or noon
or mid-afternoon
4.
Evening Prayer
5.
Night Prayer (and a Marian Antiphon)
6.
Office of Readings – at any time or anticipated on previous evening (I think there’s a few liturgical days when it can’t be anticipated) and I believe it can’t be the last office of the day, I think that has to be Night Prayer.