Hi, we just recently started using mikrotik routers for public hotspots, we currently use the 951, Basebox2 and just started testing with the GrooveA.

Since, I'm relatively new to wireless systems other than the basic home stuff, is there a way I can optimise either of those routers for smaller devices such as cellphones, tablets and laptops to get more range from them?

Currently I can get about 100 meters with my phone using the GrooveA and 12dBi Vertical Omni antenna using out of the box settings.

Gain is at 0dB and tx is at router default.

Regards
MB

The problem is not in mikrotik. You have a very high power device - GrooveA and your antenna is enough even for 2km range, but the antennas in cellphones are very small so you can't get more range with them.
Your clients - the tablets and cellphones will see the AP, but the AP will not be able to hear them.

I figured that would be the issue. As a laptop down the street, ~500m could see the signal but no chance in that happening to a phone.

Is there any other set up tips you can give me, like is 20 or 20/40 better, best Mhz for signal, etc?

20/40 is better only if you need high speeds. 20Mhz will be more stable for longer distance.
For frequency just a pick a channel that is most clear of noise.