Not difficult actually unless you have absolute zero idea of the term used in the modem setup

From the feature advertised with the WiFi 5, basically it was basically covered most of the feature that WiFi 6 has. Most modems were released with future features yet to be made official by the IEEE. Usually before any new features are made official, the draft is well known to manufacturers.

New technologies introduced with 802.11ac include the following:
Extended channel binding
--Optional 160 MHz and mandatory 80 MHz channel bandwidth for stations; cf. 40 MHz maximum in 802.11n.

More MIMO spatial streams
--Support for up to eight spatial streams (vs. four in 802.11n)

Downlink multi-user MIMO (MU-MIMO, allows up to four simultaneous downlink MU-MIMO clients)
--Multiple STAs, each with one or more antennas, transmit or receive independent data streams simultaneously.
--Space-division multiple access (SDMA): streams not separated by frequency, but instead resolved spatially, analogous to 11n-style MIMO.
--Downlink MU-MIMO (one transmitting device, multiple receiving devices) included as an optional mode.

Modulation
--256-QAM, rate 3/4 and 5/6, added as optional modes (vs. 64-QAM, rate 5/6 maximum in 802.11n).
--Some vendors offer a non-standard 1024-QAM mode, providing 25% higher data rate compared to 256-QAM

Other elements/features
--Beamforming with standardized sounding and feedback for compatibility between vendors (non-standard in 802.11n made it hard for beamforming to work effectively between different vendor products)
--MAC modifications (mostly to support above changes)
--Coexistence mechanisms for 20, 40, 80, and 160 MHz channels, 11ac and 11a/n devices
--Adds four new fields to the PPDU header identifying the frame as a very high throughput (VHT) frame as opposed to 802.11n's high throughput (HT) or earlier. The first three fields in the header are readable by legacy devices to allow coexistence
sos https://en.wikipedia.org/wiki/IEEE_802.11ac-2013

The most significant feature is Uplink MU-MIMO. However, in home environment or most home users only made quest on the uplink. So, there will be nothing significant in uplink MU-MIMO.
If you need to pay to replace the WiFi router, I'd say just skip it.

Do note that China modem for China market and for international market most likely different. China regulation in transmit power is lower than allowed in Malaysia. So, the range will be affected due to lower power.