I would like to create an EOIP between 2 points.
Site A has dynamic IP through xDSL and it makes use of DDNS
Site B is using 4G to connect and it is behind CGNAT.

Is it possible to create an EOIP between Site A & B, please? I yes, how?

EoIP needs a static IP address for both end points. CGNAT rules that out in your case.

Your best option is to create a VPN link to the DDNS router.

Or if you're running 7.1RC2 and your devices are ARM devices then you could try Zerotier.

If the IPs at both sites are dynamic, you'll always have some short-term interruption whenever one of the addresses changes. If you don't mind, and you don't mind using some DDNS system to publish the current public IP address of Site A, you can manually configure IPsec with a responder at Site A and initiator at Site B, and use it to carry the EoIP.

Or, if you don't need the L2 tunnel to carry VLAN - tagged frames, you can use L2TP/IPsec, also with server at Site A and client at Site B, and instead using EoIP, activate BCP at both ends (by indicating a bridge in /ppp profile linked to the username at both devices).