Air carrier First Officer reported they descended below the glidepath crossing restrictions on an RNAV approach.

Narrative

We were flying the RNAV Runway XX into ZZZ on a clear VMC day; and I was the pilot monitoring. Our approach clearance from ATC was 'Cross ZZZZZ at or above 9000; you are cleared for the RNAV XX into ZZZ.' During the preflight planning the captain inserted a disconitinuity between ZZZZZ in our flight plan and ZZZZZ on the approach to protect us from flying the approach without an approach clearance. We temperature compensated the fixes on the approach except for ZZZZZ1 and ZZZZZ in accordance with the briefing guide. This would leave ZZZZZ at a hard 9000 feet which is what we set for the ZZZZZ intersection linked to the approach; however we did not put the hard 9000 foot crossing restriction on the ZZZZZ intersection in our flight plan which was the fix we were direct to at the time. The default VNAV profile the aircraft had calculated to cross ZZZZZ would've been around 16000. Therefore; when we received the approach clearance and dropped ZZZZZ on ZZZZZ; the VNAV profile indicated a steep drop ahead. This is because the airplane was set up to cross ZZZZZ at 16000; and by connecting the approach to the flight plan; the airplane would then try to cross ZZZZZ at 9000. It became very clear at that moment; we had a very high energy state. On the leg approaching ZZZZZ2; the Captain used the FPA mode to descend aggressively whilst using various forms of drag to slow the aircraft down on speed profile. This was working well; and he also momentarily applied TCS to slow the rate of descent momentraily to avoid an EGPWS warning with the mountain just short of ZZZZZ2 given our rate of descent. The mountain was visible; and we were able to maintain what appeared to be a safe distance visually. However around this time; I noticed that in our haste to descend; we had never returned the aircraft to VNAV mode; and the aircraft was still descending aggressively in FPA. This removed all protections for meeting crossing restrictions on the approach. I saw we were below the VNAV profile; and I told the Captain this at 8000 feet. He responded by increasing FPA to a positive value; and the aircraft began to level off and then climb again. The lowest I recall seeing the altimeter go was 7800 feet before climbing again; however the chart stated that we should've been at least 8600 feet at that point in time. The aircraft then being leveled from the descent was properly configured for landing and on final approach speed prior to ZZZZZ2 in accordance with the briefing guide; so we continued the approach. The rest of the approach and landing was uneventful. Once on the ground; the Captain and I debriefed what had happened; and how we could do better next time. The discontinuity between both ZZZZZs and the decision not to put a hard 9000 foot crossing restriction on both ZZZZZs is what caused the aggressive descent in the first place and the chain of events thereafter. I believe there was a component of high workload and distractions at play due to the following reasons: The aggressive descent in a manual mode would give us very little time to recapture VNAV. I was simultaneously sequencing the single engine missed approach procedure into the MCDU after contacting tower in accordance with the briefing guide. We were not initially given a landing clearance on initial contact but rather asked to report ZZZZZ2 adding another layer of complexity to communications. I was making many configuration changes to the aircraft in a relatively short timeframe to assist the captain in losing energy fast; and I was devoting a large amount of attention to crosschecking no limitations were exceeded since at the point we would've been rushed. I believe my unfamiliarity with the ZZZ Airport may have lowered my situational awareness to an extent. I believe that my failure to announce 'course alive' earlier while established on a published leg of the approach may have affected the mindset of the Captain. If I would have said course alive earlier; The Captain likely would'veshifted his instrument scan to confirm the current course. It's logical that soon after; he would also likely check the vertical profile. This would've been an additional reminder to check our descent profile and may have prevented us from blowing through the VNAV path. Next time I would fix the programming in the MCDU so that both ZZZZZs when joined would make a standard VNAV descent. I would be more attentive to the flight path of the aircraft as pilot monitoring and consider delaying my radio call and programming of the MCDU until I was positive the aircrafts trajectory was as planned. I would also make sure to make my pilot monitoring callouts in a more timely manner so that an additional layer of protection is in place to prevent overshooting. I think airport specific training for first officers would be beneficial.