Hi,
I've enabled these sources on my adblock package:
My adblock version is 3.5.5-2 with total domains blocked: 138236
Ads on apps like MX Player, Podcast Addict & 9gag are still showing.
What's a recommended source to block ads on those apps ?
Be aware that you will never be able to squash all ads with dns based blocklist. What might help you depends entirely on your environment/ applications/ usage scenarios and how seriously your 'opponents' from the ad- and content industry want to fight you - respectively how much blocking your router hardware allows (you can easily OOM yourself, if you go overboard - even on devices with considerably more than 128 MB RAM).
Effectively you'll have to experiment what's needed to make the internet bearable, there is no one-size-fits-all solution here (and what has worked yesterday might no longer do tomorrow).
Use the app blokada on the phone
I would recommend using nextdns.io. No consumer router can handle a list of the size needed to block most of adds.
An R7800 with 256MB of RAM with a 100K list would exhibit it periodic 100% CPU utilization (dnsmasq) several times a minute. The longer the list, the longer those overload events would last. That would happen when there was still 50% of RAM available, so there would be other issues way before any OOM’s.
Have you considered pi-hole? Not something that can run on the router I think but very effective in protecting everything on the LAN.
I know about pi hole but I just don't have raspberry pi or any other SBC to configure it
The best thing about it is that I can use It on any device even if it is not connected to my router. Ad blocking on the router is very limited, because it does not solve the problem of mobile devices. The router has been performing way better since I disabled the adblock service.
Yes that's true, do you have any alternatives to nextdns.io ? just in case the service is down or it becomes a fully paid service
According to simple-adblock I have 105656 domains blocked on my WRT3200ACM (in reality, that number is even higher, because it uses the same "compression" code as adblock to remove unneeded third/fourth-level domains) and I've never experienced issues you're talking about.
There's a special Mobile Ad Trackers list: https://cdn.jsdelivr.net/gh/jawz101/MobileAdTrackers@master/hosts which I've had to disable myself because there were too many false positives in that list, but it may help OP.
I only have GL.iNet 6416 with 16MB of ROM and 64MB of RAM
Also R7800 here with a 300k list but with unbound, never seen such high periodic CPU utilisation here. R7800 has 512MB RAM.
It will become paid and I will happily pay for it when it does.
I understand that with enough effort and/or hw I can make it work on LAN, but that solution does not solve the problem of mobile devices when they leave the home network. Nextdns solves that problem and lets me have the same curated filtering for all devices no matter where they are.
Well, maybe I'm a bit biased ... 
... but all my mobile devices are always connected via wireguard to my local, adblock enabled dns. Works flawlessly as well ...
Yes and I have been down this road. The effort required to setup and maintain this for several dozen devices is way more than netxdns.io will charge for their services. Then I also have to treat my router as a service and coordinate restarts and upgrades. I cannot wait for nextdns to start changing because that will indicate that they will stick around for a while.
With all due respect and as much as I appreciate your efforts, there are backwards incompatible changes made in adblock once in a while and I personally gave up on trying to keep up. The last one that made me disable the adblock completely was a major version upgrade in 19.07 (a stable release, btw). On top of that, some sources change their URLs periodically, and so on. With nextdns I no longer have to worry about these things.
Default simple-adblock lists will run on routers with less RAM than yours and you'd still notice dramatic change in number of ads. You can then experiment with adding more lists if you want to see how much your router can handle. If you want proper RAM management I suggest you install adblock.
So I suggested Blokada. Have you actually had a look at it?
I use this on my phones and it works flawlessly. It eliminates pretty much all ads and there's a wide list of available lists to choose from. It does not require rooting your phone.
I prefer using ad blocking solutions on the client device rather than on the router.
I've used blokada but I have a potato phone so sometimes the vpn gets terminated and I have to open blokada again to activate it
There's an alternative to nextdns.io it's https://adguard.com/en/adguard-dns/overview.html