I have this model of router with 1 Gbit/s internet connection from my provider, what is the maximum speed I can get on ports from eth2-5? I'm asking because I've found on the net that it's passive PoE, so if I want to connect an AP even supporting the WIFI6 standard, I'll still get max speed 100Mbit/s

Reading the Specifications is out of date nowadays...
https://mikrotik.com/product/RB750UPr2

What are you saying is out of date? The hEX lite line is a 10/100 router.

What are you saying is out of date? The hEX lite line is a 10/100 router.

I think what rextended wanted to point out is that reading the specifications is what most people should do before buying a device or start asking questions about it.
Quite a lot of questions which should not have to be asked if this was done upfront.

RTFM

Beware that upgrading to the gigabit version of this product isn't a guaranteed fix. PoE + gigabit is tricky.

For one, if all parties aren't obeying the specs, you can fall down to 100 Mbit/sec there, too.

For another, I wouldn't consider the hEX PoE a gigabit router. Instead of replacing your current hEX PoE Lite, I'd leave it in place for doing PoE duties where 100 Mbit/sec connections suffice (e.g. IPcams, VoIP phones...) and add a gigabit-capable router to it, connected via the fiber link. Now you've got PoE + gigabit in a combo that won't fall down on you.

EDIT: I made an unwarranted assumption that because this product is in the "hEX" line it had six ports like the others, the last being an SFP. Alas, it does not.

@holvoetn: exactly...