Confirm. When copying files from HDD connected to the router:
[ 4215.953773] ------------[ cut here ]------------
[ 4215.958415] WARNING: CPU: 2 PID: 0 at net/sched/sch_generic.c:467 0x806deba4
[ 4215.965448] NETDEV WATCHDOG: eth0 (mtk_soc_eth): transmit queue 0 timed out
[ 4215.972373] Modules linked in: ksmbd(O) nf_nat_amanda nf_conntrack_amanda nft_fib_inet nf_flow_table_ipv6 nf_flow_table_ipv4 nf_flow_table_inet wireguard snd_usb_audio rndis_host nft_reject_ipv6 nft_reject_ipv4 nft_reject_inet nft_reject_bridge nft_reject nft_redir nft_quota nft_queue nft_objref nft_numgen nft_nat nft_meta_bridge nft_masq nft_log nft_limit nft_hash nft_fwd_netdev nft_flow_offload nft_fib_ipv6 nft_fib_ipv4 nft_fib nft_dup_netdev nft_ct nft_counter nft_compat nft_chain_nat nf_tables nf_conntrack_netlink mt7615e(O) mt7615_common(O) mt76_connac_lib(O) mt76(O) mac80211(O) lz4 libblake2s ipt_REJECT cfg80211(O) cdc_ether xt_time xt_tcpudp xt_multiport xt_mark xt_mac xt_limit xt_comment xt_TCPMSS xt_LOG vhci_hcd usbnet usbip_host usbip_core usbhid ums_usbat ums_sddr55 ums_sddr09 ums_karma ums_jumpshot ums_isd200 ums_freecom ums_datafab ums_cypress ums_alauda ts_kmp ts_fsm ts_bm snd_usbmidi_lib sch_cake ntfs3(O) nfnetlink_queue nfnetlink nf_reject_ipv4 nf_nat_tftp nf_nat_snmp_basic
[ 4215.972669] nf_nat_sip nf_nat_pptp nf_nat_irc nf_nat_h323 nf_log_ipv4 nf_flow_table nf_dup_netdev nf_conntrack_tftp nf_conntrack_snmp nf_conntrack_sip nf_conntrack_pptp nf_conntrack_irc nf_conntrack_h323 nf_conntrack_broadcast nf_conntrack_bridge macremapper(O) lz4_decompress lz4_compress libchacha20poly1305 libblake2s_generic iptable_mangle iptable_filter ip_tables hwmon hid_generic crc_ccitt compat(O) br_netfilter fuse cls_bpf act_bpf sch_tbf sch_ingress sch_htb sch_hfsc em_u32 cls_u32 cls_tcindex cls_route cls_matchall cls_fw cls_flow cls_basic act_skbedit act_mirred act_gact sg hid evdev ledtrig_gpio cryptodev(O) nf_log_ipv6 nf_log_common ip6table_mangle ip6table_filter ip6_tables ip6t_REJECT x_tables nf_reject_ipv6 nfsd bonding ip6_gre ip_gre gre ifb nat46(O) sit ipcomp6 xfrm6_tunnel esp6 ah6 xfrm4_tunnel ipcomp esp4 ah4 ip6_tunnel tunnel6 tunnel4 rpcsec_gss_krb5 auth_rpcgss oid_registry tun snd_rawmidi snd_seq_device snd_pcm_oss snd_mixer_oss snd_hwdep snd_compress snd_pcm snd_timer
[ 4216.060245] snd soundcore ovpn_dco(O) xfrm_user xfrm_ipcomp af_key xfrm_algo lockd sunrpc grace isofs hfsplus hfs cdrom autofs4 dm_mirror dm_region_hash dm_log dm_crypt dm_mod dax nls_utf8 nls_koi8_r nls_cp866 nls_cp1251 zram algif_skcipher algif_rng algif_hash algif_aead af_alg echainiv arc4 leds_gpio xhci_plat_hcd xhci_pci xhci_mtk xhci_hcd ahci libahci libata gpio_button_hotplug(O) xfs f2fs ext4 mbcache jbd2 btrfs xor raid6_pq mii tpm rng_core crc32_generic
[ 4216.188511] CPU: 2 PID: 0 Comm: swapper/2 Tainted: G O 5.10.101 #0
[ 4216.195969] Stack : 00000000 00000000 80c40000 8237d040 00000000 00000000 00000000 00000000
[ 4216.204310] 00000000 00000000 00000000 00000000 00000000 00000001 8140fd18 80aa9114
[ 4216.212643] 8140fdb0 00000000 00000000 8140fbd8 00000038 8040ce84 ffffffea 00000000
[ 4216.220982] 8140fbc8 0000076b 80a45108 ffffffff 8140fcf8 808b823c 00000000 808c0000
[ 4216.229320] 00000009 00000002 ffffffff 8237d040 00000000 804a82ac 00000008 80c20008
[ 4216.237660] ...
[ 4216.240099] Call Trace:
[ 4216.240204] [<8040ce84>] 0x8040ce84
[ 4216.246114] [<804a82ac>] 0x804a82ac
[ 4216.249585] [<8000a184>] 0x8000a184
[ 4216.253055] [<8000a18c>] 0x8000a18c
[ 4216.256532] [<803f0fc4>] 0x803f0fc4
[ 4216.260002] [<8008afd8>] 0x8008afd8
[ 4216.263471] [<8008afd8>] 0x8008afd8
[ 4216.266939] [<806deba4>] 0x806deba4
[ 4216.270407] [<800355cc>] 0x800355cc
[ 4216.273885] [<806deba4>] 0x806deba4
[ 4216.277356] [<800356c4>] 0x800356c4
[ 4216.280845] [<806deba4>] 0x806deba4
[ 4216.284351] [<843dc9cc>] 0x843dc9cc [mac80211@8f9e4cae+0x745b0]
[ 4216.290240] [<806de8a0>] 0x806de8a0
[ 4216.293712] [<806de8a0>] 0x806de8a0
[ 4216.297208] [<800ac528>] 0x800ac528
[ 4216.300682] [<800af7d8>] 0x800af7d8
[ 4216.304152] [<800ac7c4>] 0x800ac7c4
[ 4216.307623] [<806428ec>] 0x806428ec
[ 4216.311093] [<80093040>] 0x80093040
[ 4216.314571] [<800ac9e0>] 0x800ac9e0
[ 4216.318042] [<8008d98c>] 0x8008d98c
[ 4216.321512] [<8089f884>] 0x8089f884
[ 4216.324980] [<8040f090>] 0x8040f090
[ 4216.328453] [<8003a3c8>] 0x8003a3c8
[ 4216.331923] [<8040e8cc>] 0x8040e8cc
[ 4216.335401] [<80003c68>] 0x80003c68
[ 4216.340373] ---[ end trace 0f8d976d6b60321e ]---
[ 4216.344993] mtk_soc_eth 1e100000.ethernet eth0: transmit timed out
[ 4216.351493] mtk_soc_eth 1e100000.ethernet eth0: Link is Down
[ 4216.384005] mtk_soc_eth 1e100000.ethernet eth0: configuring for fixed/trgmii link mode
[ 4216.392220] mtk_soc_eth 1e100000.ethernet eth0: Link is Up - 1Gbps/Full - flow control off
Also, as I told before, something wrong with the FTP protocol. Copying from HDD is very slow and with transmitting timed-out errors. Uploading to a remote FTP server is buggy too.
P.S. Buggy VLAN patchset can be fixed with this patch.