You can try seeing if you can set the speed/duplex of NIC/ports manually if auto-detection keeps getting it wrong.

Unifi I like the APs for mesh & multiple SSID+vlans but I keep them on dedicated vlan with zero internet access because I don’t trust that I properly followed instructions to disable opted in analytics/telemetry. The mgmt software is alright but new UI wastes a lot of space. The PoE switch was alright until it stopped being able to keep a config last year. USG router I kept less than a year because it was too slow with any useful features enabled. I’ve glanced around at replacement APs here & there but pretty much waiting until I have more wifi 7 compatible devices and that’ll be another couple years.

I had issues with DNS checks and traced it to my pihole. I changed that container’s resolv.conf to use cloudflare DNS and it has been working fine since. It was with Caddy so needed to change over to use IPs.

They changed quite a few things between DSM 6 & 7 and unfortunately one of them broke easy use of those USB sticks. I didn’t want to mess with the internal config of the Synology NAS too much so used the VM approach with the HA image and mapped the USB stick to it.