Very slow speed on Linksys 1900ac 5GHz behind ISP ADSL router

I have installed Linksys 1900ac with OpenWRT 19.07 linked by cable to my ISPs Technicolor TC7230 .
I check my internet speed with
When I'm not using the Linksys and I connect directly through wifi to the ISP router the speed is around 100Mbps.
But when I connect through the Linksys, no matter if it is by Wifi (802.11nac 5.180 GHz Bitrate:520 Mbit/s)
or by cable, the speed drops to less than 30Mbps.
I'm using OpenVpn/NordVPN and NextDNS on my Lynksys, but they are not cause of the problem because even if I disconnect them the speed keeps in the 30Mbps range.
I'm no expert, is there something in my configuration of the Lynksys OpenWRT that could be causing the drop in speed?