@dakorres 

Yeah you have a mix of generations there, the 110v2 devices wont work on the SDNv4 as you say.   Honestly, the EAP110 is old technology in general and if you are considering replacement you should be looking at a EAP225 / 245 or newer

EAP110 v2 have not received any firmware update since 2018 . Maybe TP-Link Company has stopped supporting these devices ? Then why invest in TP-Link when their devices become unsupported in a few years ?

In answer to your question, the EAP110v2 is a WiFi 4 (802.11N) grade device and therefore now classified as legacy hardware by all vendors.  The original release date for the v2 if memory services me correctly was Nov 2014.  Given that was 6 years ago its more than acceptable to no longer offer support on this model. 

This is not a TP Link only thing, Uniquiti also drop support for older hardware with every new release.   It would be totally unfeasible to support older, arguably less secure hardware as newer and more advanced technology comes out to replace it.    To do so not only slows the rate of development but could also reduce the security of the solution as older "less robust" hardware and solutions would still be supported.

SDNv2.6 was dropped from support last year, that basically means the EAP110 was supported for 5 years which in technology is a lifetime.   3 years is the average cycle, 5 is pretty good to be honest.

If you bought a new Sky Q box for 4k, is it Sky fault you have a 5 year old non 4k TV that wont support it?  just being honest..

@Philbert Thanks a lot for your kind answer.