Here"s a pretty comprehensive list of modern miniatures wargames, with a description of each ruleset:
https://www.hntdaab.co.uk/blog/wargaming/great-big-modern-wargaming-rules-comparison-complete-edition
I've played Spectre Operations from that list. It's pretty simple at heart - roll a
d6, add a skill number and try to beat the target's defence number+
d6 with some modifiers. If you hit, roll a
d6 against a target number for the weapon to see if the target is killed instantly (modifiers for body armour). If your guy survives, roll a
d6 against a table to see how injured you are.
There's the complication of lots of specialist gear giving extra rules and modifiers, but in your scenario the troops may just have a rifle. Although the focus of the game is special forces, the rules and stats would allow several squads of trained soldiers to fight a similar force, or go up against a mob of untrained militia or even cartel troops.
For a third-world country, you may be looking at older tech and there are more rulesets.
AK-47 Republic by Peter Pig is precisely what you're looking for. Second edition is a tidied up ruleset, but first edition has more entertaining third-world events and political "complications". It's based on 15mm figures, but you can decide whether to look at 28s.
Too Fat Lardies has Troops, Weapons and Tactics or I Ain't been Shot Mum (company-sized games) with several modern supplements. My favourite is B'Maso, a really good look at African wars of independence and guerilla movements from the 50s to the 70s.
TFL also sells a game called Charlie Don't Surf, a game of asymmetrical warfare in the Vietnam conflict.
Lots of Cold Wars games, but they tend to focus on larger battle actions with smaller figures (6mm, 10mm or 15mm).
Plenty of manufacturers make minis in different scales - your side of the pond, Rebel Minis make nice modern figures in 15mm, Footsore Miniatures make them in 28mm.
Empress Miniatures sell a lot of modern forces in 28mm,and a lovely line of 28mm modern vehicles.
Caesar Miniatures do 20mm figures that fit with a lot of HO/OO model railway scenery, although their modern troops and insurgents may have been rotated out of production. It's a scale (1:72) where you can get a lot of model tank kits, helicopters and so on.
Grubby Tanks here in the
UK sells the old Britannia line of 20mm moderns in metal, with a range of modern and Cold War vehicles in resin.
Lots of mini manufacturers do Vietnam figures/vehicles and African or SE Asian scenery/buildings. You can make a passable shanty town yourself just from corrugated card.
Have fun!